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

Libraries/Text/RCTTextField.h

@@ -20,8 +20,11 @@
 @property (nonatomic, strong) UIColor *placeholderTextColor;
 @property (nonatomic, assign) NSInteger mostRecentEventCount;
 @property (nonatomic, strong) NSNumber *maxLength;
+@property (nonatomic, assign) BOOL textWasPasted;
 
 - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher NS_DESIGNATED_INITIALIZER;
+
 - (void)textFieldDidChange;
+- (void)sendKeyValueForString:(NSString *)string;
 
 @end