Expose SourceInfo on Map object

[mikemorris]

Need some way to grab center, bounds and similar keys from SourceInfo to build a getInfo Tilelive API method similar to Vector.prototype.getInfo.

[jfirebaugh]

I think this wants the center and zoom from the style (mapbox/mapbox-gl-style-spec#247, not implemented yet). Grabbing it from the source is weird because there can be multiple sources.

[mikemorris]

Yea, looking at Mapnik this info seems to be tied directly to the XML stylesheet. I was thinking that we would somehow composite the parameters from multiple sources together, but implementing this on the style itself would be much simpler.

[jfirebaugh]

We're holding on building out the tilelive API for GL -- it doesn't have a clear use case.