Add support for pit.only

Add support for pit.only; to match it.only().

Also, a common mistake is to forget to return a Promise. Currently the test would just silently pass without actually testing anything. Fail loudly instead.
Note that I just merged in https://github.com/jeffmo/jasmine-pit/pull/4 which runs the promise builder with the same context as `it` -- so I think you'll need to rebase here.

(my bad)