Add option for Vagrant development

Vagrant development was deprecated in favor of a metal installation per issue #51 (closed). This raises the barrier to entry for potential contributors. Issue #54 (closed) was opened to bring Vagrant back as one development option.

This MR adds a Vagrantfile that creates a box suitable for local development. All processes are run by the "vagrant" user, hopefully sidestepping any problems the arise from using two different accounts.

The configuration is largely the same as the Vagrantfile originally included with the cookbook, but it removes the "git" user, performs checks for required plugins, adds an option to disable port forwarding, and fixes some ruby permission warnings with synced directories. It also adds a simple gitlab command alias to start the service.

This has been tested on Mac OS X v10.9.