Add test coverage checking

[nowtv-web]

For instance, Istanbul (https://github.com/gotwarlost/istanbul).

Please define minimum thresholds for statements, branches and functions.

[Nighthawk14]

Istanbul is only running locally. We can implement https://coveralls.io/ which is free for open-source (as travis) and you can have coverage history by build etc..

It allows you to put a badge as well on your readme with the coverage percentage. :)

[rmallols]

@Nighthawk14 That tool looks really good. Nice spot!

[Nighthawk14]

Just implemented coveralls.io in #12.

I set up the coverage threshold for failure at 80% and the coverage decrease alert at 5%
That create comments on PR which can be check before merging, and I think prevent the merge if the coverage decrease is lower than the setup (5%).

[rmallols]

Looks really good, thanks @Nighthawk14