Add support for /v2/events stream

[nuclon]

The PR allows to use event stream from Marathon REST API added in v0.9.0

[solarkennedy]

Hmm. This is kinda new and different. Can you add an itest for it?
Also setup.py probably needs to change.

[solarkennedy]

Thank you. Long term, if you need this feature to continue to work as marathon evolves, it needs to have a section in the actual itest code that actually tries to read the events and maybe asserts that there is like, something?

But this technically isn't a blocker to merging.

My last request is that you rebase, I merged in a bunch of pep8 fixes.

[nuclon]

Hi,
I've added actual itest code. I take some time to make it working on mac, but it works now.

[solarkennedy]

Ok, the last thing to do is to bring it up to pep8 compliance. Check out the flake8 issues in an example travis run: https://travis-ci.org/thefactory/marathon-python/jobs/123588467

Or run tox -e pep8

[nuclon]

Tests are failing on marathon waiting timeout.

[nuclon]

It looks like the feature isn't really supported till 0.11.0 due to d2iq-archive/marathon#1926
so I updated tests to run starting from 0.11.0 and above

[solarkennedy]

This looks great to me. Thank you very much for iterating through the tests and pep8 and all the things. Thanks!