[NavigatorIOS] Allow translucent on NavigatorIOS

Summary:
Hi,

I've updated the NavigatorIOS component to allow setting the translucent property.

usage is:
```
            <NavigatorIOS
                translucent={false}
            />
```

This is my first contrib to react-native, so apologies if I've missed something.

Cheers,
Owen
Closes https://github.com/facebook/react-native/pull/1273
Github Author: Owen Kelly <owen@novede.com>

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

Libraries/Components/Navigation/NavigatorIOS.ios.js

@@ -57,6 +57,7 @@ var RCTNavigatorItem = createReactNativeComponentClass({
     backButtonIcon: true,
     backButtonTitle: true,
     tintColor: true,
+    translucent: true,
     navigationBarHidden: true,
     titleTextColor: true,
     style: true,
@@ -300,6 +301,11 @@ var NavigatorIOS = React.createClass({
      */
     titleTextColor: PropTypes.string,
 
+    /**
+     * A Boolean value that indicates whether the navigation bar is translucent
+     */
+    translucent: PropTypes.bool,
+
   },
 
   navigator: (undefined: ?Object),
@@ -609,6 +615,7 @@ var NavigatorIOS = React.createClass({
           navigationBarHidden={this.props.navigationBarHidden}
           tintColor={this.props.tintColor}
           barTintColor={this.props.barTintColor}
+          translucent={this.props.translucent}
           titleTextColor={this.props.titleTextColor}>
           <Component
             navigator={this.navigator}