Flask-mixins

- Bugfix for SchemaMixin not dumping responses in the tuple format with a status code
- Tests for the Resource and Resources mixin added
- Werkzeug response object returned without modifications (in some cases the user may want to return a manual response with custom headers or other attributes)

Features:
- Permissions added
- BaseMiddleware added

Fixes:
- SchemaMixin no longer converts None response to {}, the conversion should be done by another Mixin (JsonifyMixin)

Fixes:
- StatusCodeMixin explicit None check

- Flask version requirement decreased

Initial release.

Features:
- PermissionMixin
- StatusCodeMixin
- JsonifyMixin
- SchemaMixin
- ResourceView
- ResourcesView