Merge pull request #356 from elasticsales/simpler-cls-query

Simplify _cls queries when only a single class
Added comment about why temp debugging exists
Merge branch 'master' of github.com:MongoEngine/mongoengine
Code cleanup
Filter out index scan for pymongo cache
Doc fix for #340
Fixed hashing of EmbeddedDocuments (#348)
ImageFields now include PIL error messages if invalid error (#353)
Updated AUTHORS (#340, #348, #353)
Explicitly set w:1 if None in save
Reverting Fixed hashing of EmbeddedDocuments (#348)
Removing old test re: #348
list_indexes and compare_indexes class methods + unit tests
better comment
merge feature-index-comparison into closeio