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 don't have my machine in front of me, but I believe Play services is contained in the SDK component "Google Repository" - if so, that is the component that should be listed here.