Move App to Heroku

 * [x] create a Heroku application
 * [x] tell Herkou about domain: `heroku domains:add ahab.io`
 * [x] enable PG on Heroku: `heroku addons:add heroku-postgresql`
 * [x] add Honeybadger API token to Heroku env: `heroku config:set HONEYBADGER_API_KEY=...`
 * [x] export DB from MySQL on Linode
 * [x] import DB to PostgreSQL on Heroku: `heroku pgbackups:restore DATABASE 'http://ahab.io/pgdump.txt' --confirm ahabio` (file subsequently removed)
 * [x] set `$stdout.sync = true`
 * [x] change `config/database.yml` and `Gemfile` to use PostgreSQL
 * [x] use `img` helper for `search_glass2x.png` so it uses the right URL in production
 * [x] use a Heroku-compatible asset system
 * [x] add @svizzari as collaborator on the Heroku app
 * [x] CNAME ahab.io to ahabio.herokuapp.com (after merge)