Source modal

![msource](https://cloud.githubusercontent.com/assets/83384/2939174/90fc8c74-d93a-11e3-99eb-a0a9a8ef4575.gif)

- Removes `Add Mapbox` source button and flow
- Only displays local sources in History pane
- Sources are changed in a style via modal that
  - Splits local/remote sources into two lists
  - Provides a textfield for adding remote sources by mapid manually

Refs https://github.com/mapbox/tm2/issues/45#issuecomment-42432980