Add coverage support

[kuraga]

[kuraga]

@arqex

1. You can also add Coveralls support. I can't because registration needed.

2. try npm run cober. You'll see:

-----------------|-----------|-----------|-----------|-----------|
File             |   % Stmts |% Branches |   % Funcs |   % Lines |
-----------------|-----------|-----------|-----------|-----------|
   freezer/      |       100 |       100 |       100 |       100 |
      freezer.js |       100 |       100 |       100 |       100 |
   freezer/src/  |        88 |     83.33 |     89.47 |      88.1 |
      emitter.js |     55.17 |      37.5 |        50 |     55.17 |
      freezer.js |       100 |      87.5 |      87.5 |       100 |
      frozen.js  |     91.03 |     90.59 |     96.97 |     90.99 |
      mixins.js  |      93.1 |     84.38 |       100 |      93.1 |
      utils.js   |     78.72 |        55 |     73.33 |     79.55 |
-----------------|-----------|-----------|-----------|-----------|
All files        |     88.06 |     83.33 |     89.47 |     88.16 |
-----------------|-----------|-----------|-----------|-----------|

So, we have some coverage problems.

[arqex]

This is really interesting, I need some time to have a deeper look at it. The rest of badges, I don't see a benefit on having them.

Thanks for your suggestions @kuraga

[kuraga]

Coveralls is not for a badge but for a message about coverage (example). But I agree with you - it's not very useful.