Introduce bower dependency managment for frontend libraries

cc @th3fallen @DeepDiver1975 @PVince81 

This was wanted in https://github.com/owncloud/core/issues/10220#issuecomment-51841522

Is this the way we want to manage 3rdparty libs or is this still not the perfect way to go?

This introduces the special "app" bower for `addStyle` and `addScript` to properly resolve the paths to the vendor path. See f4756d311e2d437110c8e282b970b389ce11413f

Opinions? 

**Edit**

* [ ] update documentation about the new shortcuts
@th3fallen  not to sure if I understand your concern - composer will be used to pull php libs into 3rdparty - whereas bower will pull js/css resources into core/vendor.

I see no conflict in here - or do I miss anything