Add coverage support

[Gil Forcada Codinachs]

To grow confidence on our code and be able to refactor the code base some coverage tools need to be put in place, so we know how much code is still not tested.

coveralls.io for example has integration with travis, so it's perfect for us.

I already integrated that on a personal project, so, if nobody beats me I will do so later.

[Leandro Regueiro]

@gforcada No problem at all. I like the idea.

[Gil Forcada Codinachs]

Already done