infinitescroll: adding wrapped ability to block on thenable

If the callback function handed to infinitescroll is a promise
generator, then the binding is naturally debounced until the promise
resolves.

It is important to note that if the promise is rejected, that binding
will never fire again.
infinitescroll: retval check and debug messages
Merge pull request #10 from behance/infinitescroll

Infinitescroll: debounced based on callback return