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.
I did some testing on this. I can't get this branch to build without the Google Repository but Google Play services appears to be somewhat optional. https://developer.android.com/sdk/installing/adding-packages.html however states that both Google Repository & Google Play services should be installed to use Google Play services.