Add Support and information for MySql DB

MySql is the db used when deploying Cloud Foundry onto AWS using AWS Bosh Bootstrap.

1. I have added gem dependency for `mysql2` in the `Gemfile`.
2. Added `libmysqlclient-dev` as Ubuntu Prerequisite library.
3. Add more information related to MySql in the readme.

I have tested this thoroughly on Ubuntu 12.04 LTS.