Installing Nginx w/ FastCGI.

I've added Nginx to the provisioning script and configured FastCGI to redirect to HHVM. Chose to go with Nginx rather than Apache on the recommendation of a couple people in the IRC channel, plus it has FastCGI without needing any additional modules. I'm hoping that the simpler installation process will make Nginx the easier of the two options to maintain as HHVM evolves.

Thanks for putting your Vagrant build on GitHub, by the way – I'm just getting started with HHVM and this was quite helpful.

Resolves #3