Use info from getlasterror whether a document has been updated or created.
More precise "created" keyword argument signals

If a document has a user given id value, the post_save signal always got the
"created" keyword argument with False value (unless force_insert is True).

This patch uses the result of getlasterror to check whether the save was an
update or not.
Merge branch 'getlasterror' of github.com:helduel/mongoengine into getlasterror

Conflicts:
	mongoengine/document.py
Merge branch 'getlasterror' of https://github.com/helduel/mongoengine into 163
Updated authors and changelog #163