Added placeholderTextColor property for TextInput class

Summary:
Closes https://github.com/facebook/react-native/pull/997
Github Author: Maksim Bunkow <bimawa@gmail.com>

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

Libraries/Components/TextInput/TextInput.js

@@ -445,6 +445,7 @@ var TextInput = React.createClass({
           onSubmitEditing={this.props.onSubmitEditing}
           onSelectionChangeShouldSetResponder={() => true}
           placeholder={this.props.placeholder}
+          placeholderTextColor={this.props.placeholderTextColor}
           text={this.state.bufferedValue}
           autoCapitalize={autoCapitalize}
           autoCorrect={this.props.autoCorrect}