Marathon 0.7.0 support and partial refactor

Summary:
* Added support for Deployments, Groups, native Docker containers, Queue, Server Info, Server Metrics, and Ping
* Updated object attributes to be at parity with Marathon 0.7.0 (RC2)
* Updated return types to be at parity with Marathon 0.7.0 (RC2)
* Updated method signatures to be more consistent
* HTTP 4xx errors other than 404 now through `MarathonHttpError` instead of `NotFoundError`
* `json_encode()` and `json_decode()` on MarathonResource have been renamed to `to_json()` and `from_json()`, respectively
* Refactored serialization/deserialization to reduce boilerplate

Test Plan: Create/update/delete each resource type

Reviewers: merkkila

Reviewed By: merkkila

Subscribers: tberman, peter, justin, stephan, serdar

Differential Revision: http://phabricator.thefactory.com/D3408