prepare for node-mapnik@1.3.x which no longer auto-registers datasources
skip EINVAL to allow browsing a virtual box shared directory on windows guest back to OS X
windows fixes for latest node-mapnik
minor fixes to multilayer handling hack - refs #2210
Revert "travis is now running latest node v0.10.x so remove nvm hack"

This reverts commit e7c5c549a83b095112e399644d5fabb8725c6afd.

Conflicts:
	.travis.yml
don't install postgres since travis already has it