Port to NAN

[springmeyer]

Need to port node-mapnik to use NAN in order to gracefully develop against both node v0.10.x and v0.12.x.

[springmeyer]

@rvagg and @kkoopa - do you know of any experienced NAN developers who might be interested in tackling this port? Contracting is a possibility.

[rvagg]

Pulling in @thlorenz here as he's interested in getting up to speed on NAN

[kkoopa]

Does not appear to be using any exotic stuff, so porting should be rather straightforward. nodejs/nan#50 might help, although it has not been updated for NAN 1.0.0.

[springmeyer]

done thanks to @kkoopa as part of #256