[WebView] Exposed scalesPageToFit UIWebView property

Summary:
Added the ``scalesPageToFit`` prop to ``WebView``. This allows ``UIWebView`` to handle user zoom and scale.
Closes https://github.com/facebook/react-native/pull/1631
Github Author: alvaromb <amedina@apsl.net>

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

Libraries/Components/WebView/WebView.ios.js

@@ -95,6 +95,11 @@ var WebView = React.createClass({
      * Used for android only, JS is enabled by default for WebView on iOS
      */
     javaScriptEnabledAndroid: PropTypes.bool,
+    /**
+     * Used for iOS only, sets whether the webpage scales to fit the view and the
+     * user can change the scale
+     */
+    scalesPageToFit: PropTypes.bool,
   },
 
   getInitialState: function() {
@@ -155,6 +160,7 @@ var WebView = React.createClass({
         onLoadingStart={this.onLoadingStart}
         onLoadingFinish={this.onLoadingFinish}
         onLoadingError={this.onLoadingError}
+        scalesPageToFit={this.props.scalesPageToFit}
       />;
 
     return (