Makes 'db' argument to connection optional.
Merge pull request #737 from wcdolphin/master

Make 'db' argument to connection optional
Update changelog.rst
fix-#746: Stop ensure_indexes running on a secondaries unless connection is through mongos
fix-#397: Allow specifying the '_cls' as a field for indexes
Merge pull request #719 from DavidBord/fix-397

fix-#397: Allow specifying the '_cls' as a field for indexes
Change with PR #719 and close mongoengine/Issue #397
Fix merge of PR #747