Add support for coveralls.

[xordoquy]

Not sure it'll work out of the box, let's see with a PR.

[xordoquy]

Note that this PR is linked with #2936

[jpadilla]

@xordoquy there's also pytest-cov

[xordoquy]

@jpadilla right, but I need to update first runtest.
I wonder why we don't go straight with py.test command line nowadays

[xordoquy]

https://coveralls.io/builds/2965226

[thedrow]

Just FYI there's also http://codecov.io which has a bit more features like failing the build when coverage decreases.
Doesn't matter much though.

[tomchristie]

@thedrow Looks nice. Thanks for noting that.

[xordoquy]

@tomchristie I'd like your feeling on f19c145

[tomchristie]

I'd like your feeling on f19c145

@xordoquy I think ideally my preference in all of this would be to treat #2936 as a pre-requisite to any coverage service integration. It'd be nicer if our travis builds did something that's as close as possible to how we run tests locally. (IE yeah using runtests is preferable, really)

[tomchristie]

Let's stay with #2936 being a pre-requisite for integrating any coverage service.
I'd rather see us add this in in an agnostic way first, and then make an informed choice on which, if any, service we should integrate that with.