Implements onKeyPress

Summary: - When a key is pressed, it's `key value` is passed as an argument to the callback handler.
 - For `Enter` and `Backspace` keys, I'm using their `key value` as defined [here](https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/key#Key_values). As per JonasJonny & brentvatne's [suggestion](https://github.com/facebook/react-native/issues/1882#issuecomment-123485883).

- Example
```javascript
 _handleKeyPress: function(e) {
      console.log(e.nativeEvent.key);
  },

  render: function() {
    return (
      <View style={styles.container}>
        <TextInput
            style={{width: 150, height: 25, borderWidth: 0.5}}
            onKeyPress={this._handleKeyPress}
        />
        <TextInput
            style={{width: 150, height: 100, borderWidth: 0.5}}
            onKeyPress={this._handleKeyPress}
            multiline={true}
        />
      </View>
    );
  }
```
- Implements [shouldChangeCharactersInRange](https://developer.apple.com/library/prerelease/ios/documentat
Closes https://github.com/facebook/react-native/pull/2082

Reviewed By: javache

Differential Revision: D2280460

Pulled By: nicklockwood

fb-gh-sync-id: 1f824f80649043dc2520c089e2531d428d799405

Examples/UIExplorer/TextInputExample.ios.js

@@ -42,6 +42,7 @@ var TextEventsExample = React.createClass({
       curText: '<No Event>',
       prevText: '<No Event>',
       prev2Text: '<No Event>',
+      prev3Text: '<No Event>',
     };
   },
 
@@ -51,6 +52,7 @@ var TextEventsExample = React.createClass({
         curText: text,
         prevText: state.curText,
         prev2Text: state.prevText,
+        prev3Text: state.prev2Text,
       };
     });
   },
@@ -73,12 +75,16 @@ var TextEventsExample = React.createClass({
           onSubmitEditing={(event) => this.updateText(
             'onSubmitEditing text: ' + event.nativeEvent.text
           )}
+          onKeyPress={(event) => {
+            this.updateText('onKeyPress key: ' + event.nativeEvent.key);
+          }}
           style={styles.default}
         />
         <Text style={styles.eventLabel}>
           {this.state.curText}{'\n'}
           (prev: {this.state.prevText}){'\n'}
-          (prev2: {this.state.prev2Text})
+          (prev2: {this.state.prev2Text}){'\n'}
+          (prev3: {this.state.prev3Text})
         </Text>
       </View>
     );