Rails back-end experiment

An experimental version of Streetmix that runs on Ruby/Rails rather than Node/Express. 

My main reason for trying this (besides familiarity on my part) is to switch the data store from Mongo to Postgres [#446] ActiveRecord is a more complete ORM than any of the existing Node options.

Not ready for merging, but ready for testing at: http://streetmix-rails.herokuapp.com