Add map tiling support (merge orta/NAMapKit)

[dblock]

We've been working off a fork of NAMapKit for the Artsy iPhone app (http://iphone.artsy.net, try the Armory Show). That code diff is orta/NAMapKit@neilang:master...master. It will take a bit of effort to merge. Most importantly it supports image tiling.

I think there're really two things:

• Generalize annotations, we hacked NAPinAnnotationView, but we should extract the annotations into something that supports a protocol so that ours can be its own thing.
• Rebuild tiling on top with a working example.

@orta who wrote a lot of this code will comment here

@neilang There hasn't been much development here for a bit, can we count on you to be looking at PRs?

[neilang]

Thanks @orta and @dblock, I think this is a great feature to add. There is a lot of code there so I will try and review it this weekend and see how we can merge it in.

@tonyarnold and @donny can also accept PRs, so between all 3 of us I think we can get it added.

[dblock]

Thanks @neilang. I'll do the work, no stress :) Stay tuned, first PR today.