issue 621/async promise chunking

@gitnik @cthrax 

I chose bluebird as it is known to be faster, I also got tired of trying to make $q work on specs. All the rootscope.$apply()'s were having no effect on digest cycles. Maybe you guys can tell me where to correct it? Otherwise we gain an external dependency.

please review.
chunking is optional, it can be a pure promise as well