Add Helpful Aliases

When working with Homestead, you'll be usually working in a different directory than Homestead itself, meaning that `vagrant up` etc won't work. This adds 3 helpful aliases for people to start/stop/restart Homestead.

Credit where credit is due, I got this from [Gary Taylor](http://thereluctantdeveloper.com/2014/06/controlling-laravel-homestead-vm-from-any-folder-in-terminal), but I think it's a worthwhile addition to the docs (especially with the vm alias there already).