Change semantics of the SIGNAL_COMPLETE to run after result is stored.
Change semantics of the SIGNAL_COMPLETE to run after result is stored.

Previously the SIGNAL_COMPLETE would be sent before the result had
actually been placed in the result store. The docs indicate that the
signal is emitted when a task completes successfully, and completion
should imply that the result is ready.

Fixes #449

pombredanne/huey,coleifer/huey,rsalmaso/huey