Fixed issue with choices validation when they are simple list/tuple,
after model.validate() did not get any error message.
Added test to ensure that model.validate() set the correct error
messages.
Merge branch 'validation-message'
Merge branch 'master' of https://github.com/malderete/mongoengine into 214

Conflicts:
	AUTHORS
	mongoengine/base.py
	tests/fields/fields.py
Merge fix tests
Fixed invalid choices error bubbling (#214)