Add zIndex support

Summary:
Adds zIndex support :)

**Test Plan**

Tested the following components by adding two of each, overlapping them, and setting a high zIndex on the first of the two:

ActivityIndicator
Image
MapView
Picker
ScrollView
Slider
Switch
Text
TextInput
View
WebView

Tested on Android 4.1 and iOS 8.4. Also tested updating zIndexes on Views in my own app.

<img width="359" alt="ios activityindicator" src="https://cloud.githubusercontent.com/assets/4349082/15633473/88f842cc-257b-11e6-8539-c41c0b179f80.png">
<img width="330" alt="android activityindicator" src="https://cloud.githubusercontent.com/assets/4349082/15633475/88f95784-257b-11e6-80c0-2bf3ed836503.png">
<img width="357" alt="ios image" src="https://cloud.githubusercontent.com/assets/4349082/15633474/88f93d80-257b-11e6-9e54-4ff8e4d25f71.png">
<img width="340" alt="android image" src="https://cloud.githubusercontent.com/assets/4349082/15633478/88fd2788-257b-11e6-8c80-29078e65e808.png">
<img width="342" alt="android picker" src="ht
Closes https://github.com/facebook/react-native/pull/7825

Differential Revision: D3469374

Pulled By: lexs

fbshipit-source-id: b2b74b71d968ebf73ecfd457ace3f35f8f7c7658

Examples/UIExplorer/ViewExample.js

@@ -93,7 +93,7 @@ var ZIndexExample = React.createClass({
       flipped: false
     };
   },
-  
+
   render() {
     const indices = this.state.flipped ? [-1, 0, 1, 2] : [2, 1, 0, -1];
     return (
@@ -128,7 +128,7 @@ var ZIndexExample = React.createClass({
       </TouchableWithoutFeedback>
     );
   },
-  
+
   _handlePress() {
     this.setState({flipped: !this.state.flipped});
   }