Support exporting mbtiles interactivity layer

[onyxfish]

We need this to be able to host our tiles on S3 and still take advantage of the interactivity layer.

[springmeyer]

Just added a first cut at dumping out this data. Basically when an mbtiles file is created the utf grid json gets broken out a bit, so I re-assemble when dumping out to the file system. The one catch here is that the WAX library expects the usage of a jsonp callback called grid to be used when loading the json grids. If you are loading grids from the filesystem rather than a server then its not easy to add this jsonp support so I've simply wrapped the entire json in the callback when dumping out :)

See eaf58ed#L1R234. Ideas for improvements are welcome.

[onyxfish]

Great! We've frequently coded static jsonp callbacks into things, so I wouldn't worry too much about that. ;-) I have a potential use case for this coming up shortly as well, so I'll let you know how it goes!

Thanks!