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.
For some of the reasons apparent on this thread, I've prefered the two-step deploy process of:

```bash
git push staging master
staging migrate
```

Or:

```bash
git push production master
production migrate
```

Or:

```bash
git push production master
```

Or:

```bash
production maintenance:on
git push production master
production migrate
production maintenance:off
```

Different deploys for the same app may require varying sets of commands. Sometimes it makes sense to migrate and others, not. Sometimes it makes sense to go into maintenance mode and others, not.

The one time two commands always need to be run together is when we migrate the database, we want to immediately restart the dynos. There's where the Parity gem shines.

It would be nice to have a `bin/deploy` standard but I don't think the varying needs per-deploy support its existence.