Refacto - Pure angular

Hi 

Following #27 and #28 here is another proposal ! A more verbose one.
I personally prefer this one 'cause it allow a lot of customization [Demo](http://plnkr.co/WfnnOc)

In short before :
```xml
<ui-slider ng-model='_' ></ui-slider>
```
After
```xml
<ui-slider class="ui-slider-default">
  <ui-slider-track>
    <ui-slider-thumb ng-model='_'></ui-slider-thumb>
  </ui-slider-track>
</ui-slider>
```

[Here is a README](https://github.com/douglasduteil/ui-slider/blob/pure-angular/README.md)

I'm not done making tests. For now I just skip the scenario tests (will fix them later)
Give me feed back !
Bye