Use Vagrant for blog development

I was having weird Ruby issues upgrading RDiscount to the newest version, probably related to the upgrade to OS X Mavericks. I couldn't figure them out after a fair amount of struggle, and would like to avoid these issues in the future, so am now doing all development inside of a Vagrant VM. The commands to set it up are pretty simple, but this should ensure more consistency over time while also preventing me from making a mess of other things on my computer.