Force com.android.support:appcompat-v7 dependency to 20.0.0

The newest appcompat (21.x) does not support uses-sdk:minSdkVersion
less than 14.

For developers having the latest appcompat available, the build will
fail with + as the version number for com.android.support:appcompat-v7.
Update gradle version to 0.12.+

The current version of Android Studio does not support 0.11.+, but
updating this to 0.12.+ makes the project work in Android Studio.
Merge branch 'fix-appcompat-dependency'
Merge branch 'update-gradle-plugin-version'