Added the possibility to change the parent of the progressbar

Added the setParent function which accepts an HTMLElement. This function will remove the progressbar container from its current parent and add it to the new one. I found the need of this when developing using Ionic and needed to have the bar inside a ion-content directive. Also added the getDomElement function which returns the dom element. Respective unit tests included.

Will update the documentation if this gets merged.
I have a bad feeling about 'parent', because it will presumably pollute the global space. That is a general code issue though....