Add support for pit.only(), parity with it.only()
Require pit() tests to return a Promise

Previously if you forgot to return the promise, it would succeed
silently (without properly testing functionality)
Use jasmineEnv.currentSpec instead of `this`

This avoids any weirdness with binding runPitTest, and is more
consistent with how .runs() and .waitsFor() were being called.
Merge pull request #6 from dalongi/master

Add support for pit.only(); require pit() tests to return a Promise