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.

Opinions?