easy deploy to heroku button

[techieshark]

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

[tbuckl]

question: how does the deploy to heroku button work? can we configure it to have different database (paid and not paid)? thinking about using github deploy for testing (free) and then production (paid?).

[techieshark]

@buckleytom AFAIK there isn't much possibility of that kind of configuration. I think that once we completely eliminate the PostGIS requirement we can make the default heroku-postgresql in the addons in app.json be the free version instead of standard-0.