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? 
That's how it looks like with all files
```
treecore/vendor/moment   
core/vendor/moment
├── benchmarks
│   └── clone.js
├── bower.json
├── CHANGELOG.md
├── LICENSE
...
├── min
│   ├── locales.js
│   ├── locales.min.js
│   ├── moment.min.js
│   ├── moment-with-locales.js
│   └── moment-with-locales.min.js
├── moment.js
└── README.md
```