feature: sortable file-list in overview

A litte update of the `templates/overview.html` adding four buttons to sort the file-list by `sloc`, `complexity`, `bugs`, `lint-errors` using `jQuery`. Some tests using `casper.js`. are included.

_Edit_
I can't get the `travis` build to succeed because of some _weird errors_ which are probably related to `casperjs`.
Output:

```bash
Aborted due to warnings.
npm ERR! weird error 3
npm ERR! not ok code 0
```

_Edit_
To avoid 'notification-noise' i'll close this until i've got travis-integration up and running.