Add `bin/deploy` for Heroku deployments

Heavily influenced by [the blog's
bin/deploy](https://github.com/thoughtbot/robots.thoughtbot.com/blob/92b5c4da00a74979c62002ee4685cbb36cfd0649/bin/deploy),
`bin/deploy {staging,production}` will push to `{staging,production}` and run `rake db:migrate`.

`bin/deploy` will default to staging, but `bin/deploy $REMOTE` accepts an
arbirtrary git remote.
Not sure if this is still true, but I think it is:

> You may find that you have issues sometimes that I call “stuck dynos”. This is where not every process seems to be aware of new columns. Restarting your Heroku app will fix this problem.

http://robots.thoughtbot.com/less-painful-heroku-deploys