Add `bin/deploy` script

* Help avoid forgetting to run `rake db:migrate`.
* Help avoid forgetting to run `restart` after migrate.
* Allow each app to edit these deploy instructions to fit their
 environments' needs (such as `run rake purge` to clear a cache)
 but maintain the same `./bin/deploy` interface and convention
 that is consistent with our many Middleman and Rails apps.
`git symbolic-ref` may fail, but probably won't. I mention it only because if it *did* fail, you'd invoke

```
git push x :master
```

Which I *think* deletes the master branch. I'm on the fence on how paranoid to be about this, but thought I'd mention it.

If you *do* want to guard against this, then I think we're back to the question about `set -e`. I have a lower threshold than @mike-burns when it comes to `set -e` and would probably move back to it rather than incorporating the assignment of `branch` into the `&&` chain. I don't feel strongly though.