Vagrant support, optional build firmware w/ image builder, update dev dependencies.

With this pull request we aim to make getting started with OpenWireless dead simple. To achieve this we have:

* Added a Vagrantfile to build a virtual machine to do development in.
* Add support for building the firmware with ImageBuilder (building from source in a virtual machine is took 3hrs+), building with image builder from cold takes about 8min, and with artifacts cached takes about 3.5min)
* Add xvfb/firefox to run selenium tests inside virtual machine.

We re-used the ```install-dev-dependencies.sh``` to provision the virtual machine to get consistency between folks running with and without the VM.

Each commit message elaborates on why it was required.
Fixed. Thanks for the FYI on nounset.