Add a method that returns BFTask

[npahucki]

I recently started using the signatures available in the Parse API that return BFTasks which are a really good way to chain together asynchronous logic. I use your very helpful library in a few of my apps, and it was getting awkward to keep having to switch between BFTasks and block callbacks, so I added a signature that works with BFTask and make the library consistent with other Parse APIs.

One Note: since a BFTask can only be completed once, this signature will fail an assertion if you try to use cache-then-network policy which would try to complete the task twice.