Added preinstall check for Xvfb

When going to sleep last night, I realized we can do even better for all `node_modules` which depend on this. I have added a `preinstall` script which verifies that `Xvfb` is installed and errors out otherwise.

```sh
$ npm install

> headless@0.1.1 preinstall /home/todd/github/node-headless
> test -n "$(which Xvfb)" || (echo 'Xvfb not found. Please install it and add it to your PATH.' && exit 1)

Xvfb not found. Please install it and add it to your PATH.
npm ERR! headless@0.1.1 preinstall: `test -n "$(which Xvfb)" || (echo 'Xvfb not found. Please install it and add it to your PATH.' && exit 1)`
npm ERR! `sh "-c" "test -n \"$(which Xvfb)\" || (echo 'Xvfb not found. Please install it and add it to your PATH.' && exit 1)"` failed with 1

```

If you do merge this one, can you update the version and publish to `npm`?

Thanks a bunch!