fix-#734: set attribute to None does not work (at least for fields with default values). Solves #735 as well
fix-#771: OperationError: Shard Keys are immutable. Tried to update id even though the document is not yet saved
Better BaseDocument equality check when not saved

When 2 instances of a Document had id = None they would be considered
equal unless an __eq__ were implemented.

We now return False for such case. It now behaves more similar to
Django's ORM.
Merge pull request #798 from DavidBord/fix-771

fix-#771: OperationError: Shard Keys are immutable. Tried to update id e...
Fix merge in tests/document/instance.py
PEP8 compliance tests/document/instance.py
Fix merge in docs/changelog.rst
Merge branch 'DavidBord-fix-734'
Fix merge MongoEngine/mongoengine#799
Added merge to changelog.rst
Merge branch 'aericson-better_basedocument_eq'
Bumped django versions and added 1.7.1.
Fixed build matrix.
Exclude Django 1,7 and Python 2.6 since Django 1.7 doesn't support 2.6.