feat(progressbar): refactor/expose `bar` directive & refactor `progress`

Since the Bootstrap 3 changed the markup we should expose the inner bar so user can define the the type. (This is compatible and with v2)

Except that, now we can support text, custom markup and elements inside progress / bar elements, like what we we were trying to do in #725.

Also, removed superfluous handlers and stacked bars helpers, that made the directive's usage & code inflexible and probably nobody was using :-)

Anyway, I think this is a much cleaner approach.

Before merging, there is a pending issue, probably an AngularJS bug. If I use `templateUrl` for the `progress`, then the `bar` cannot find the required controller. To avoid this we can either use inline `template` or the `progress.html` should have more than one elements. For example: 
````html
<span><div class="progress" ng-transclude></div></span>
````