Make some settings optional

This commit makes the ELASTICSEARCH_URL and REDIS_URL settings optional.
If they're not set, then Fracas will try to use the URL provided by the
corresponding Heroku addon. This means less setup when installing on
Heroku. Likewise, REDIS_PASSWORD is obsoleted. Instead, any Redis auth
keys are automatically parsed from your Redis URL.
Heroku button
Add deployment instructions to README
Make image minification optional

It breaks too much
Fix reference to ng-grid