[feature] add Menu model, update POST methods for Restaurants

story:

> As a Restaurant Owner, I want to be able to add menus when creating my restaurant in one action so that I dont have to add Menus **only** after creating my restaurant.

changes:
- added Menu model (as an Embedded Document)
- updated all models to DynamicDocument or DynamicEmbeddedDocument instead [allowing flexibility](https://mongoengine-odm.readthedocs.org/guide/defining-documents.html#dynamic-document-schemas)
- added colander Schemas for Menu, and updated colander Schema for Restaurant (linking them up)
- updated POST requests on Restaurants so that users can create a restaurant with menus right away
- refactored `serialize` hook so that serializing to JSON is now more robust (cover Embedded Documents)
- improved test cases, updated tests
- improved tests
- remove coveralls.io badge for now