Integrating code coverage tool Blanket.js

Hi @dschaffe

I'm sorry for taking this on my own back, but I've done a first sweep of integrating Blanket.js (a code coverage tool for unit tests) into the awsome extension you've made.

The change is very straightforward, 

Firstly to add code coverage in one of the included files you simply append "*" to the end of that file, for example:

```javascript
/* brackets-xunit: includes=jquery.js,mySource.js*,underscore.js,raphael.2.0.1.js */
```

... this means you don't have to see how many lines of jQuery, underscore or raphael you failed to cover - very useful!

I've added the necessary unit test to parseIncludes.

Blacket.js works for qunit out of the box, and I believe it's possible to add adapters for the other test runners.

Can you let me know if you are likely to include this in the extension, if so I'll carry on with the other adapters!