[TextInput] returnKeyType, enablesReturnKeyAutomatically, secureTextEntry, more keyboardTypes

Summary:
This diff completes adding support for the following UITextField properties:

- returnKeyType: what the return key on the keyboard says
- enablesReturnKeyAutomatically: the return key is disabled iff there is no text entered. This too could be implemented in React but it feels better in UIKit right now because it is handled synchronously.
- secureTextEntry: obscure passwords
- keyboardType: added all the keyboard types, they are useful in different scenarios

There were varying degrees of support for these properties so it looks like this diff continues some unfinished work. I also updated the keyboardType enum to use native constants instead of strings like the other properties here.

Added examples to the UIExplorer.
Closes https://github.com/facebook/react-native/pull/265
Github Author: James Ide <ide@jameside.com>

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

Examples/UIExplorer/TextInputExample.js

@@ -174,6 +174,89 @@ exports.examples = [
       );
     }
   },
+  {
+    title: 'Keyboard types',
+    render: function() {
+      var keyboardTypes = [
+        'default',
+        'ascii-capable',
+        'numbers-and-punctuation',
+        'url',
+        'number-pad',
+        'phone-pad',
+        'name-phone-pad',
+        'email-address',
+        'decimal-pad',
+        'twitter',
+        'web-search',
+        'numeric',
+      ];
+      var examples = keyboardTypes.map((type) => {
+        return (
+          <WithLabel key={type} label={type}>
+            <TextInput
+              keyboardType={type}
+              style={styles.default}
+            />
+          </WithLabel>
+        );
+      });
+      return <View>{examples}</View>;
+    }
+  },
+  {
+    title: 'Return key types',
+    render: function() {
+      var returnKeyTypes = [
+        'default',
+        'go',
+        'google',
+        'join',
+        'next',
+        'route',
+        'search',
+        'send',
+        'yahoo',
+        'done',
+        'emergency-call',
+      ];
+      var examples = returnKeyTypes.map((type) => {
+        return (
+          <WithLabel key={type} label={type}>
+            <TextInput
+              returnKeyType={type}
+              style={styles.default}
+            />
+          </WithLabel>
+        );
+      });
+      return <View>{examples}</View>;
+    }
+  },
+  {
+    title: 'Enable return key automatically',
+    render: function() {
+      return (
+        <View>
+          <WithLabel label="true">
+            <TextInput enablesReturnKeyAutomatically={true} style={styles.default} />
+          </WithLabel>
+        </View>
+      );
+    }
+  },
+  {
+    title: 'Secure text entry',
+    render: function() {
+      return (
+        <View>
+          <WithLabel label="true">
+            <TextInput secureTextEntry={true} style={styles.default} value="abc" />
+          </WithLabel>
+        </View>
+      );
+    }
+  },
   {
     title: 'Event handling',
     render: function(): ReactElement { return <TextEventsExample /> },