Use elevation to implement shadows on Android

Summary: This PR includes a working interface to Android's `elevation` property implemented as an RN style. Elevation is the only (easy) [platform-supported way to create shadows](http://developer.android.com/training/material/shadows-clipping.html). For it to work note that you must be running on Android 5.0+, and add `elevation` to a view with a `backgroundColor` set. These are platform limitations.

This PR is not intended to be merged in its current state, but rather to inform the discussion from #2768. At a minimum, before merging we would need to add the elevation style to the docs and rebase this to master (**EDIT** I have now rebased on master because from v0.14.2 too many commits were being pulled in -- haven't tested it since the rebase though). Additionally, it might be good to add tests, although I couldn't find any for the Android code. I'm happy to get that done if this feature gets signed off by the React Native team.

Finally, as I argued in #2768 I think that `elevation` is a useful abstraction ov
Closes https://github.com/facebook/react-native/pull/4180

Reviewed By: svcscm

Differential Revision: D2684746

Pulled By: mkonicek

fb-gh-sync-id: 825f3ccd20c4b0eea9d11b5f0e3a6b018b7e4378

Libraries/Components/View/ViewStylePropTypes.js

@@ -47,6 +47,14 @@ var ViewStylePropTypes = {
   ),
   shadowOpacity: ReactPropTypes.number,
   shadowRadius: ReactPropTypes.number,
+  /**
+   * (Android-only) Sets the elevation of a view, using Android's underlying 
+   * [elevation API](https://developer.android.com/training/material/shadows-clipping.html#Elevation).
+   * This adds a drop shadow to the item and affects z-order for overlapping views.
+   * Only supported on Android 5.0+, has no effect on earlier versions.
+   * @platform android
+   */
+  elevation: ReactPropTypes.number,
 };
 
 module.exports = ViewStylePropTypes;