Node mapnik 1.4 (ships binaries)

node-mapnik@1.4.0 is now installable via binaries for OS X and Linux 64 and soon for Windows (likely both 32 bit and 64).

This means that `npm install` now works without mapnik or libprotobuf being installed externally.

This pull updated to node-mapnik@1.4.0 and adapts documentation to be clear that an external mapnik is no longer needed for these common platforms where binaries are provided.