Mapnik 3.x

It's time: this pull moves master to targeting Mapnik 3.x, and therefore bumps the node-mapnik version to 3.x too.

  - Only supports c++11 compilers
  - Ships with new set of optimized binaries only supporting latest Linux (14.04) and OS X (10.9)
  - New 1.x branch will stay compatible with Mapnik 2.3.x and non c++11 capable compilers and platforms (https://github.com/mapnik/node-mapnik/tree/1.x).

/cc @artemp @BergWerkGIS @yhahn