Updated HEROKU.md squashed

I've added an extra step to explicitly add the production.rb file.

Added S3 CDN instructions with asset sync gem. Users assets will auto sync with S3 on deploy, using Gziped files where available. This should significantly help with Heroku stability under load.

Added add-on instructions for some load testing services available through Heroku. Hopefully help us hack on performance and stability issues.

Some general houskeeping, fixed h1 h2 muddle and removed bundle exec from foreman commands, as it uses bundler by default, and added some commands for running tests.