Add Vagrantfile

Because of the zeromq dependency, I think it's a lot easier for folks who want to try ruby-trade just to run a VM with both a server and client running. Attaching a quickly tested Vagrantfile.

In a follow-up commit, I've also updated README.md to streamline the instructions for installing and running ruby-trade for the first time. Feel free to not use that commit if you disagree with these changes.