New source UI features

@ajashton @ian29 @samanpwbb  

New source UI features added based on this discussion https://github.com/mapbox/tm2/pull/418 

Included features:
- Update layer names
- Refresh layers (if the original file has been changed)
- Add layers more easily
- Add multiple layers from single source
- Autofill projection
- Projection field is editable in Postgis and SQLite sources
- Zoom-to-layer

![tm2_example](https://cloud.githubusercontent.com/assets/1209162/3237749/163d8cde-f0ed-11e3-8d04-14cdd22dee99.gif)

Let me know if you have any suggestions, concerns, questions, etc.