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