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 https://github.com/owncloud/documentation/pull/636
@karlitschek I got asked if then `vendorScript` and `venderStyle` could be backported to stable7. This would enable stable7 apps to use it. (@LEDfan asked)