making swagger-ui location relative when schemes and host are not provid...

...ed

This makes it possible to omit the host and scheme and have it adopt the host and scheme of the docs url. Handy when your API docs follow the API through lots of envs.  (e.g. local, dev, test, stage, prod) 

Will require someone to verify I didn't break something unexpectedly.

![noidea](https://cloud.githubusercontent.com/assets/754053/4687590/49386264-5658-11e4-8267-2f3c3114101b.jpg)