Modify Vagrantfile to use a custom gittip image.

The use case for this change is to allow multiple new devs to
easily set up clean new dev environments over a slow or non-existent
network. (This actually happened at Pycon 2014). The instructions for
creating this image have been added as comments in the readme file, and
quick start instructions have been added to the readme file. The online
URL for the image is http://downloads.gittipllc.netdna-cdn.com/gittip.box