IDE support

I created an [Android Studo plugin](https://github.com/evant/android-studio-unit-test-plugin) which requires a gradle plugin to provide a model for the ide to read. I don't expect you to merge this pull request as-is because it dependes on a local library that provides the interface between the plugin and the ide. This [library](https://github.com/evant/android-studio-unit-test-plugin/tree/master/android-unit-test-model) needs to be hosted seperatly on maven central so that the gradle plugin can be updated independently. I'm not sure if you want to pull this in and maintain it yourself, or have me do it.
Create a new testClasses that runs the classes build step for all the test variants. This is run by default from the intellij junit test runner, but could be useful independently as well.