Adds the imap_enumerated function.

Like imap, but yields tuple of original request index (as it was provided in the requests parameter sequence) and response object.

Unlike imap, failed results and responses from exception handlers that return None are not ignored. Instead, a tuple of (index, None) is yielded. Additionally, the requests parameter must be a sequence of Request objects (generators will be exhausted, but this may cause issues if the generator is infinite or otherwise very large).

The index is merely the original index of the original request in the requests list and does NOT provide any indication of the order in which requests or responses are sent or received. Responses are still yielded in arbitrary order.

Resolves #159