Use Python for secret token setting in deployment (installed by default ...

...on OSX and Ubuntu)

Addresses @migurski's comments here: https://github.com/codeforamerica/cityvoice/pull/196#issuecomment-48776143

We discussed it, and the goal here is that blindly copying/pasting the deployment commands into a terminal should Just Work™.

This doesn't solve the problem for Windows users, but it _does_ eliminate the need for a redeployer to install anything beyond Git and the Heroku toolbelt. Since we envision having a "CityVoice Provisioner" micro-app to build instances for non-technical users, this seems like the path of least resistance.

(I've tested that this method of setting the token works in production on Heroku.)