Update build tools to 20.0.0
Update Android Gradle plug-in to 0.12.+
Update target sdk version to 20
Fix dependencies

- Use aar of crouton
- Make sure to NOT use the very latest versions (21/Android L preview) of
  the android support libraries.
Remove duplicate meta-data definition
Update .gitignore
Add missing Gradle wrapper directory

This also makes gradlew executable.
Enable Travis CI build