[ReactNative] Add PerformanceLogger to measure TTI

Summary:
@public

Add PerformanceLogger to keep track of JS download, initial script execution and
full TTI.

Test Plan:
The Native side currently calls `addTimespans` when it's finish initializing
with the six values (start and end for the three events), so I just checked it
with a `PerformanceLogger.logTimespans()` at the end of the function.

```
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "ScriptDownload: 48ms"
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "ScriptExecution: 106ms"
2015-06-18 16:47:19.096 [info][tid:com.facebook.React.JavaScript] "TTI: 293ms"
```

React/Base/RCTRootView.m

@@ -17,6 +17,7 @@
 #import "RCTEventDispatcher.h"
 #import "RCTKeyCommands.h"
 #import "RCTLog.h"
+#import "RCTPerformanceLogger.h"
 #import "RCTSourceCode.h"
 #import "RCTTouchHandler.h"
 #import "RCTUIManager.h"
@@ -247,6 +248,7 @@ RCT_NOT_IMPLEMENTED(-initWithCoder:(NSCoder *)aDecoder)
 - (void)insertReactSubview:(id<RCTViewNodeProtocol>)subview atIndex:(NSInteger)atIndex
 {
   [super insertReactSubview:subview atIndex:atIndex];
+  RCTPerformanceLoggerEnd(RCTPLTTI);
   dispatch_async(dispatch_get_main_queue(), ^{
     if (!_contentHasAppeared) {
       _contentHasAppeared = YES;