[TabBarIOS] Add translucent property to TabBarIOS

Summary:
To be on par with NavigatorIOS, I added the translucent property to TabBarIOS.

Usage:
```
<TabBarIOS
 translucent={false} // default is true
/>
```
Closes https://github.com/facebook/react-native/pull/1937
Github Author: Jean Regisser <jean.regisser@gmail.com>

Libraries/Components/TabBarIOS/TabBarIOS.ios.js

@@ -32,7 +32,11 @@ var TabBarIOS = React.createClass({
     /**
      * Background color of the tab bar
      */
-    barTintColor: React.PropTypes.string
+    barTintColor: React.PropTypes.string,
+    /**
+     * A Boolean value that indicates whether the tab bar is translucent
+     */
+    translucent: React.PropTypes.bool,
   },
 
   render: function() {
@@ -40,7 +44,8 @@ var TabBarIOS = React.createClass({
       <RCTTabBar
         style={[styles.tabGroup, this.props.style]}
         tintColor={this.props.tintColor}
-        barTintColor={this.props.barTintColor}>
+        barTintColor={this.props.barTintColor}
+        translucent={this.props.translucent !== false}>
         {this.props.children}
       </RCTTabBar>
     );