Nav ios hide hairline

Summary:
For the current project I am working on I needed to add a boolean option (`shadowHidden`) to remove the 1px hairline shadow from the NavigationIOS component:

`<NavigatorIOS shadowHidden={true} initialRoute={...} />`

By default, or with `shadowHidden={false}` it looks like this:

![withhairline](https://cloud.githubusercontent.com/assets/129330/8145401/d2704956-11d4-11e5-86e8-a75435b68480.png)
Setting the shadow hidden  with `shadowHidden={true}` looks like this:

![nohairline2](https://cloud.githubusercontent.com/assets/129330/8145405/148ed56e-11d5-11e5-85d6-f8cd3453d5ac.png)

The code it uses to do the actual hiding is... a bit of a hack (*I* think), but it's the only way currently to do it, and is the way they do it in the iPhone calendar app: iterating through the subviews and removing the shadow image.

This removes the shadow *and* keeps the translucent blurry effect with a ScrollView (see this [SO discussion](http://stackoverflow.com/questions/18160173/how-to-remove-uinavigationbar-inner
Closes https://github.com/facebook/react-native/pull/1615
Github Author: Mr Speaker <mrspeaker@gmail.com>

Libraries/Components/Navigation/NavigatorIOS.ios.js

@@ -60,6 +60,7 @@ var RCTNavigatorItem = createReactNativeComponentClass({
     tintColor: true,
     translucent: true,
     navigationBarHidden: true,
+    shadowHidden: true,
     titleTextColor: true,
     style: true,
   },
@@ -281,6 +282,11 @@ var NavigatorIOS = React.createClass({
      */
     navigationBarHidden: PropTypes.bool,
 
+    /**
+     * A Boolean value that indicates whether to hide the 1px hairline shadow
+     */
+    shadowHidden: PropTypes.bool,
+
     /**
      * The default wrapper style for components in the navigator.
      * A common use case is to set the backgroundColor for every page
@@ -640,6 +646,7 @@ var NavigatorIOS = React.createClass({
           rightButtonTitle={route.rightButtonTitle}
           onNavRightButtonTap={route.onRightButtonPress}
           navigationBarHidden={this.props.navigationBarHidden}
+          shadowHidden={this.props.shadowHidden}
           tintColor={this.props.tintColor}
           barTintColor={this.props.barTintColor}
           translucent={this.props.translucent !== false}