Fixed sum and average mapreduce function for issue #375
Merge branch 'master' of https://github.com/MongoEngine/mongoengine

Conflicts:
	AUTHORS
Fixed sum and average queryset function

* Fixed sum and average map reduce functions for sum and average so that
        it works with mongo dot notation.

* Added unittest cases / updated them for the new changes
Merge remote-tracking branch 'origin/pr/376'

Conflicts:
	AUTHORS
Fixed sum and average mapreduce dot notation support (#375, #376)