Add NFS options to Vagrantfile and vagrant instructions.

Using Vagrant slows down everything: running the rails server, running rspec. I found instructions in a blog post (http://kubaf.wordpress.com/2013/01/12/vagrant-speed-up-on-mac-os-x/) about how to speed things up using NFS. I applied that and the speed was much better (10 sec vs. 2 sec for the same page load). This pull request will make NFS work "right out of the box".