Add Vagrant environment configured to run tests

Since Knex has a dependency on three different database engines, I thought it might be handy to have a Vagrant environment that ships with properly-configured MySQL, PostgreSQL, and SQLite. Thoughts?

To use, you'll need [VirtualBox](http://virtualbox.org) and [Vagrant](http://vagrantup.com) installed. Builds are available for all major platforms. Then, in the repository root, run

```bash
vagrant up
```

to spin up a new VM and provision it with Knex's supported database engines. The repository is mounted at `/vagrant` on the VM, so any changes you make on your host machine are automatically reflected in the VM.

To run tests:

```bash
vagrant ssh
cd /vagrant
npm test
```

If you guys think this is useful, I'm happy to write up some documentation. I recently removed MySQL from my laptop; figured I'd set this up rather than keep MySQL around just for the Knex test suite.