Add Styles API

[perrygeo]

https://www.mapbox.com/developers/api/styles/

[perrygeo]

I think the major design decisions for the SDK center around how much validation we want to do here.

From the HTTP resource perspective, this API is a relatively simple set of GET, PUT, POST, PATCH and DELETE methods. From the user's perspective, the most challenging part will be constructing valid mapbox GL styles.

We want to keep this low level so I'd propose that, if any GL style helpers are needed, they should be external modules. For now, maybe just assume valid styles and let the HTTP response convey any errors. What do you think @sgillies ?