Style creation from source updates for atlas

* removes ability to create style from remote source from "Create style from source" button in offline mode
* adds a field for creating a style from a custom/remote source to New Project pane (a la https://github.com/mapbox/mapbox-studio/issues/663). This is still rudimentary in execution.

![screen shot 2014-11-19 at 11 50 56 am](https://cloud.githubusercontent.com/assets/3952537/5113238/09600238-6fe3-11e4-85bc-dedaacba7489.png)