add app.json for deploy to heroku system

docs: https://devcenter.heroku.com/articles/heroku-button
add logo that works on light background
use dark logo for heroku's light background
don’t include unused deployhooks add-on

- this is used by Mesa team to post to a Slack channel,
  but we don’t want every deployment in the world to post in there.

- also, this add on needs an extra parameter (the url of the slack web hook)
  and since it wasn’t included in the config, the deploy was failing.
don't need hobby-dev db when using standard-0

we only need one database.
add deploy to heroku button
make app deployment easier

- adds new rake task app:deploy
  which in turn calls `db:migrate`, `council_districts:load`,
  and `legistar_all:refresh`.
- uses task in app.json for auto deploy and in README
add website to app.json

- Note that currently the website and repo url are
  the same but that may change eventually (if we create a github pages
  landing page for instance).
integrate deploy button into documentation
remove trailing whitespace
set stack, region in app.json

- stack must be cedar for this to work (see https://github.com/codeforamerica/MuniciPal/issues/42).
- region is almost always going to be in the US, at least for now.
not sure what the stack syntax is, maybe this?
switching to Ruby 2.1.2

- 2.1.2 is recommended on Heroku:
  https://devcenter.heroku.com/changelog-items/468

- `debugger` gem does not build on 2.1.2; switching to `byebug`
   `bye bug` is the recommended debugger for current Ruby + Rails (see https://github.com/cldwalker/debugger/issues/125#issuecomment-44028274 and https://github.com/rails/rails/issues/14533)

- The following commands may help in switching your local version (assuming you use rbenv):

	rbenv install 2.1.2
	rbenv local 2.1.2
	gem install bundler
	bundle
Heroku doesn’t support setting stack in app.json

Heroku support says: “I’m afraid our interface or the app.json format doesn't currently allow this. Our API does, but this probably wouldn't fit your use case.”
Remove failed attempt at setting app.json’s region

After testing, this didn’t seem to have any effect.
Merge pull request #86 from codeforamerica/feature/easy-deploy-to-heroku-button

easy deploy to heroku button