Intial pass at adding emoji support in markdown

Requires running `bundle exec rake emoji` to copy the emoji images
to `public/image/emoji` path.
Instrument emoji
Add rake as development dependency
Update start scripts to use foreman
Move deploy_assets to script/
Move gemoji to development dependency
Use `foreman run` in deploy script
Copy emoji locally before deploying assets
Add deploying code to deploy script
Rename deploy script
Create task to generate all available emoji
Remove `rake emoji` from deploy script
Generate emoji
Render emoji on asset host