I want to add FragmentTestUtil

I tried to add new test runner for testing fragment. But a few guys(including @xian) mentioned about add that functionality to XXXTestUtil instead of XXXTestRunner.
So I put that functionality to a new class(org.robolectric.util.FragmentTestUtil).
Please merge it or give your opinion again.

You can find my origin issue [here](https://github.com/pivotal/robolectric/pull/452)

Thanks.