Viewhelpers/media

Hi Claus,

I added:
* one abstract class for media related VHs
* one abstract class for media related tag based VHs
* one abstract class for image related VHs

plus:
* one VH for image tags inheriting from the above
* one VH for image URLs inheriting form the above
* one VH for video tags inheriting from the above

All Vhs are able to render absolute URLs if configured or in BE mode, the tag based VHs add HTML5 specific attributes and child tags can be added.

I hope I delivered ;-)