Shutdown of thread pool inside Dispatcher only if it is a PicassoExecutorService

If I want to provide an external thread pool, I don't want picasso have the shutdown operation on it.
Also changed the tests to reflect the fact that you can have an internally created or externally provided thread pool.
Capitalize "shutdown", capitalize "picasso", and put a period at the end. [Comments are sentences][1].

 [1]: http://nedbatchelder.com/blog/201401/comments_should_be_sentences.html