Add support for changing sites directory

0.2.0: Add support for changing the directory we look at for sites files

* Write nginx configuration instead of using nginx.conf from the deb
* Allow sites directory to be set via `node.nginx_passenger.sites_dir`
* Add initial serverspec tests

0.0.5: Add support for PASSENGER_APP_ENV

v0.0.5

0.0.5: Add support for PASSENGER_APP_ENV