Support for --test-runner

[Aswin Ly]

By supporting this parameter, I can define my test runner class as well, instead of the APK default. (See: https://github.com/gojuno/composer)

[Trevor Jones]

This should be pretty simple to add if someone wants to grab it. lets just be sure to update tests to cover it.

Also if anyone decides to pickup the work please leave a note here when you start so we don't duplicate effort.