Beefing up base image

Adding the following to the image:

 * Basic dev tools
 * RVM
 * Ruby 2.1 via RVM
 * PosrgreSQL client
 * Heroku Toolbelt
 * Security credential templates for SSH and Heroku CLI
 * Startup script (so that I don't have to remember that java command)

@sashah87 @rpdillon tagging two of you, since I don't really have anyone else to tag on this!
Why use CMD and not ENTRYPOINT?