Send post_delete signals immediately

In a normal relational construct, if you're listening for an event
that signals a child was deleted, you dont expect that the parent
was deleted already.

This change ensures that post_delete signals are fired immediately
after objects are deleted in the graph.
Add David Cramer to AUTHORS
Move signal disconnect into finally block
Move logic seperation as its not longer repetitive
Improve test to ensure that post_delete was actually called
Merge pull request #642 from dcramer/patch-1

Send post_delete signals immediately