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 think `C:\Users\barbeau\.AndroidStudioBeta\config\codestyles` would be better as `%userprofile%\.AndroidStudioBeta\config\codestyles`.