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
Fix multiple connections aliases being rewritten
First pass at making BaseDict and BaseList work like their native counterparts

Python's `dict()` takes one arg and optional kwargs; similarly with `list()`.
The existing `BaseDict()` and `BaseList()` constructors break that signature.
This is first pass at getting it working for the simple case of passing in a
single arg.