Fixes #458, DecimalField now ignores incorrect values until validate is called,
just like FloatField.
Expanded example of using ListFields for one-to-many relationships.

- Added an example of how to use the atomic update operations to
  pull and push to a ListField containing ReferenceFields.
fix validation for a nested DictField
Fixes AttributeError when using storage.exists() on a non-existing file.
fix broken external docs link
Merge pull request #472 from swistakm/fix/docs-map-reduce-mongodb.org-link

fix broken external docs link on map_reduce
Merge pull request #469 from jyapayne/master

Fixes AttributeError when using storage.exists() on a non-existing file.
Merge pull request #467 from adamsc64/doc_foreignkey_push_pull_example

Expanded example of using ListFields for one-to-many relationships.
Merge pull request #468 from elasticsales/fix-dict-validation

Fix validation for a nested DictField
Merge pull request #462 from bool-dev/master

Fixes #458, DecimalField now ignores incorrect values until validate is called just like FloatField
fix(session): session serializer to json
Use defaults when host and port are passed as None
Added app_label Meta attribute to MongoUser class
Added entry to AUTHORS
Test for testing invalid dict field value.
Patch for #491

Fix for issue #491, a bug in the permissions system.
Added regression test for bug with DictField and numeric keys.

When a DictField has numeric fields, BaseDocument._lookup_field doesn't
work correclty.
Fixed bug with numeric valuedict keys and BaseDocument._lookup_field().
Fixed bug for count method when _none is True 

If my queryset have elements example:

qs.all().count() => 10
q = qs.all().none()

the count in queryset "q" must be 0
q.count() => 0
Error in GenericReferenceField serialization was fixed