Some doc/ignore updates

Google Play services is required;
Also updated the documentation for style checking, to make it easier
for Mac users.
Finally, add /build/ to .gitignore.
Remove uses-sdk from AndroidManifest.xml

minSdkVersion and targetSdkVersion are overridden by what’s in
build.gradle.
Include Google Repository as well

This is required. Google Play services may not be required, but Google says you should include both and this isn't necessarily meant to be a strict requirement set anyway. This is for troubleshooting build errors.
Minor update to CONTRIBUTING

Specify %userprofile% rather than hardcode Sean's directory.
Merge pull request #153 from OneBusAway/MinorUpdates

Minor updates