Merge branch 'master' of github.com:samuelclay/mongoengine

* 'master' of github.com:samuelclay/mongoengine: (227 commits)
  Adding QuerySet(read_preference=pymongo.ReadPreference.X) and QuerySet().read_preference() method to override connection-level read_preference on a per-query basis.
  Fixing py3 compat
  Added chaining regression test (MongoEngine/mongoengine#135)
  Updated test
  Unicode fix for repr (MongoEngine/mongoengine#133)
  Allow updates with match operators (MongoEngine/mongoengine#144)
  Unicode fix reverted but can have custom validator
  Fix loop
  Updated URLField
  Added Garry Polley to contributors list
  can now use AuthenticationBackends with permissions.
  Updates to the readme
  Added CONTRIBUTING.rst
  Updated docs thanks @mitar
  Moved injection of Exceptions to top level
  Fixed reload issue with ReferenceField where dbref=False (MongoEngine/mongoengine#138)
  Improved import cache
  Fixed objectId for DBRef
  Updated travis.yml
  Fix ReferenceField dbref = False
  ...