Improve documentation about how Mapbox API & Tokens work

@ryanbaumann is already working on this.

Since Mar 2, 2018.

### Documentation and tests should:

1. How Mapbox GL JS does or does not require a Mapbox Access token.
* Mapboxgl-Jupyter uses the Mapbox GL Style Spec, an open source specification for map styles.
* If the style sheet you specify as the `style_url` does not use resources from Mapbox APIs - which means that you created a style sheet yourself from public or locally hosted map data sources - then a Mapbox Access Token is not required.
2. How user data added to a visualization layer is never sent to Mapbox.
* Mapbox GL JS renders all data used in a visualization locally on the client using [Geojson-VT](https://github.com/mapbox/geojson-vt).
* No data included as a layer from `mapboxgl.viz.*(data)` is sent to Mapbox APIs in any way.
3. What metadata about an map tile API request is sent to Mapbox.
* Creferrer, IP Address, User Agent
4. What a Mapbox Access Token does
* Associates API requests made to Mapbox with a user account
* Meters API access based on user account

### Test should:

1. Ensure that a local non-Mapbox hosted style sheet and resources work -> https://github.com/mapbox/mapboxgl-jupyter/issues/19
2. Ensure that a non-mapbox tile resource, such as ArcGIS server WMS or WFS layer served as Geojson, works appropriately in Mapboxgl-Jupyter.