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
Fix handling of HTTP 4xx responses
Support passing multiple Marathon servers to MarathonClient()

Summary:
Servers are tried in order until a non-"Connection Error" type response is received

Also included:
- Convert double-quoted strings to single-quoted

Test Plan: Test with a combination of live and dead URLs

Reviewers: merkkila, tberman

Reviewed By: merkkila, tberman

Subscribers: tberman, peter, justin, stephan, serdar

Differential Revision: http://phabricator.thefactory.com/D3434
Update sphinx docs
Fix serialization of Marathon Constraints
Increase default timeout to 10 seconds
Tweak Marathon object __repr__
Fill in task.app_id where (older) Marathon doesn't
0.6.0 release with bridge networking support

Summary:
Mesos 0.20.1- and Marathon 0.7.1-compatible release.

Changes:
* Added bridge networking support
* Use common exception for all pick-from-a-list options (`InvalidChoiceError`). `InvalidOperatorError` has been removed
* Updated `MarathonObject.__repr__` to be more useful
* Changed default HTTP request timeout from 5s to 10s

Test Plan: Test full app lifecycle

Reviewers: merkkila

Reviewed By: merkkila

Subscribers: tberman, peter, justin, stephan, serdar

Differential Revision: http://phabricator.thefactory.com/D3529
Fix broken exception import
Support app.task_rate_limit for older Marathon versions