COOK-2622 - nginx: add upstart support

This pull request adds support for upstart to the nginx cookbook. I have not altered the default behavior of the cookbook, just allowed someone to specify an init_style of upstart.

I also added attributes under node['nginx']['upstart'] that will allow for a few configurables as well as the 'passenger post-stop hack' for those that insist on running in the background.

Thanks,
Brandon