Add test coverage to testing

[gribok]

To enforce test-driven development, let's measure test coverage via Coverage.py.
The result can be added to Coveralls, which provides a free service for open source project and has a built-in integration in Travis-CI.
It would be possible to add a new quality gate into CI pipeline, that new code have to be covered with test cases.

[Kilo59]

Was going to suggest the same thing.
@gribok do you want to take this?

[gribok]

@Kilo59 Yes. Do we want to measure statement or branch coverage?

[Kilo59]

@gribok I would say statement but if you have a case for branch coverage I'm all ears.

[gribok]

@gribok I would say statement but if you have a case for branch coverage I'm all ears.

Statement coverage sounds good. Let's try.