Add automatic testing of all PRs

[Kentzo]

Let's use py.test and codecoverage (with support for branching).

It can be configured like this:

https://github.com/Kentzo/async_app/blob/master/setup.py#L15-L27
https://github.com/Kentzo/async_app/blob/master/setup.py#L50
https://github.com/Kentzo/async_app/blob/master/setup.cfg#L25-L27

[Martiusweb]

So we need pytest to enable code coverage right?

We could enable coveralls (https://coveralls.io/) to report stats in the PRs.
I think we also want to run the tests twice (once with the "default" unittest runner) to ensure everything stays compatible with it.

[Kentzo]

@Martiusweb I'm using codecov.io, here is an example: Kentzo/Power#25. Never used coveralls.io, don't know which one is better.

I do not like unittest's runner, but sure, we can run it to ensure the compatibility.