Fix submodule import

Summary: Fix the importing of marathon.models and convert to relative imports.

Test Plan:
`python setup.py install`
then, in a python console:
`from marathon import MarathonClient`

Reviewers: merkkila

Reviewed By: merkkila

Subscribers: tberman, justin, stephan, serdar

Differential Revision: http://phabricator.thefactory.com/D2197