add managed overlay class about mbxmapkit

[radven]

This is a fabulous idea.

[incanus]

I'm pushing on this a bit for two reasons (though definitely keeping it around; just trying to explain my rationale):

• Network retry code when dealing with mapID's (in order to actually fetch TileJSON) gets nasty fast. I want to think about this properly, and we're already doing some for the main map overlay as it exists.
• I'm going to assume that the general use case of this library is more entry-level/low time investment or else the full iOS SDK would likely be used. As such, getting into an assortment of overlay functionality is relatively low priority.

[incanus]

Let's still keep this relatively low priority until we clean up behaviors around general library use and the baselayer (i.e. the overlay that can replace Apple's maps).

Over at #61, I'm headed in the direction of "Let's make our online and offline layers be well-behaved MKTileOverlay subclasses". I think that will end up obsoleting the need for an MBXOverlay.

@incanus, Thoughts on closing this issue in favor of the discussion at #61?

[incanus]

Yeah, all bets are off if we're refactoring the way the basic overlay works.