easy deploy to heroku button

This fixes #42 

Important note:

* This switches to Ruby 2.1.2 (you may need to update your local environment to test this, see the commit for docs: https://github.com/codeforamerica/MuniciPal/commit/6b01dd2f748dbed374001aefdf63ad8564d18173)

It also does these things:

* Switches the debugger to since the, uh, `debugger` gem doesn't work anymore.
* Adds an app.json file to explain to Heroku how to build the package
* Adds the deploy button to the README (twice! once in the deployment instructions and once at the top since it is so easy we might as well highlight that)
* Adds an `app:deploy` rake task which calls the tasks to migrate the database and load districts + legistar; the app.json uses this one global task and I update the README to reflect this new-found simplicity.
* Adds a dark-text logo that works on Heroku's light background