Minor updates

1. Update README: Google Play services is required;
2. Update CONTRIBUTING: Also updated the documentation for style checking, to make it easier for Mac users.
3. Add /build/ to .gitignore.
4. Remove unnecessary \<uses-sdk\> from AndroidManifest.xml.
@barbeau, here is what I see in my SDK Manager:

![screen shot 2014-07-20 at 2 12 28 pm](https://cloud.githubusercontent.com/assets/150269/3638560/96c42ece-1052-11e4-8700-45f1dfdf1c77.png)