HTML Binding for Labels

I added the ability to use "ng-bind-html" for the labels on the toggle switch.
In order to do this, it now requires ngSanitize.

To make the switch use html binding, simply add a "html" attribute to the switch:

```html
Puppies or not?
<toggle-switch
    model="switchStatus"
    html="true"
    knob-label="<strong>##</strong>"
    on-label="<em>Puppies!</em>"
    off-label="<del>Puppies</del>">
<toggle-switch>
```