Update Gradle and Build Tools to latest releases

Gradle Plugin to 0.13.0, Gradle to 2.1, Build Tools to 20.0.0, Android plugin renamed to new 'com.android.application'.

This will ensure that Muzei compile and build out of the box on Android Studio 0.8.11 (which is now the minimum compatible version).
Merge pull request #76 from ianhanniballake/update_gradle

Update Gradle and Build Tools to latest releases