[Bridge] Add support for JS async functions to RCT_EXPORT_METHOD

Summary:
Adds support for JS async methods and helps guide people writing native modules w.r.t. the callbacks. With this diff, on the native side you write:

```objc
RCT_EXPORT_METHOD(getValueAsync:(NSString *)key
                       resolver:(RCTPromiseResolver)resolve
                       rejecter:(RCTPromiseRejecter)reject)
{
  NSError *error = nil;
  id value = [_nativeDataStore valueForKey:key error:&error];

  // "resolve" and "reject" are automatically defined blocks that take
  // any object (nil is OK) and an NSError, respectively
  if (!error) {
    resolve(value);
  } else {
    reject(error);
  }
}
```

On the JS side, you can write:

```js
var {DemoDataStore} = require('react-native').NativeModules;
DemoDataStore.getValueAsync('sample-key').then((value) => {
  console.log('Got:', value);
}, (error) => {
  console.error(error);
  // "error" is an Error object whose message is the NSError's description.
  // The NSError's code and domain are also set, and the native trace i
Closes https://github.com/facebook/react-native/pull/1232
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Base/RCTBridgeModule.h

@@ -17,6 +17,20 @@
  */
 typedef void (^RCTResponseSenderBlock)(NSArray *response);
 
+/**
+ * Block that bridge modules use to resolve the JS promise waiting for a result.
+ * Nil results are supported and are converted to JS's undefined value.
+ */
+typedef void (^RCTPromiseResolveBlock)(id result);
+
+/**
+ * Block that bridge modules use to reject the JS promise waiting for a result.
+ * The error may be nil but it is preferable to pass an NSError object for more
+ * precise error messages.
+ */
+typedef void (^RCTPromiseRejectBlock)(NSError *error);
+
+
 /**
  * This constant can be returned from +methodQueue to force module
  * methods to be called on the JavaScript thread. This can have serious
@@ -37,7 +51,7 @@ extern const dispatch_queue_t RCTJSThread;
  * A reference to the RCTBridge. Useful for modules that require access
  * to bridge features, such as sending events or making JS calls. This
  * will be set automatically by the bridge when it initializes the module.
-* To implement this in your module, just add @synthesize bridge = _bridge;
+ * To implement this in your module, just add @synthesize bridge = _bridge;
  */
 @property (nonatomic, weak) RCTBridge *bridge;
 
@@ -70,6 +84,26 @@ extern const dispatch_queue_t RCTJSThread;
  * { ... }
  *
  * and is exposed to JavaScript as `NativeModules.ModuleName.doSomething`.
+ *
+ * ## Promises
+ *
+ * Bridge modules can also define methods that are exported to JavaScript as
+ * methods that return a Promise, and are compatible with JS async functions.
+ *
+ * Declare the last two parameters of your native method to be a resolver block
+ * and a rejecter block. The resolver block must precede the rejecter block.
+ *
+ * For example:
+ *
+ * RCT_EXPORT_METHOD(doSomethingAsync:(NSString *)aString
+ *                           resolver:(RCTPromiseResolveBlock)resolve
+ *                           rejecter:(RCTPromiseRejectBlock)reject
+ * { ... }
+ *
+ * Calling `NativeModules.ModuleName.doSomethingAsync(aString)` from
+ * JavaScript will return a promise that is resolved or rejected when your
+ * native method implementation calls the respective block.
+ *
  */
 #define RCT_EXPORT_METHOD(method) \
   RCT_REMAP_METHOD(, method)
@@ -118,7 +152,7 @@ extern const dispatch_queue_t RCTJSThread;
   RCT_EXTERN_REMAP_MODULE(, objc_name, objc_supername)
 
 /**
- * Similar to RCT_EXTERN_MODULE but allows setting a custom JavaScript name
+ * Like RCT_EXTERN_MODULE, but allows setting a custom JavaScript name.
  */
 #define RCT_EXTERN_REMAP_MODULE(js_name, objc_name, objc_supername) \
   objc_name : objc_supername \
@@ -136,7 +170,7 @@ extern const dispatch_queue_t RCTJSThread;
   RCT_EXTERN_REMAP_METHOD(, method)
 
 /**
- * Similar to RCT_EXTERN_REMAP_METHOD but allows setting a custom JavaScript name
+ * Like RCT_EXTERN_REMAP_METHOD, but allows setting a custom JavaScript name.
  */
 #define RCT_EXTERN_REMAP_METHOD(js_name, method) \
   - (void)__rct_export__##method { \