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!
Both can be used, but CMD gives more convenient syntax. This allows to run a container without specifying a command, but also allows to specify a command: `docker run apartmentlist/jenkins bash -l -i` to get a container with an interactive shell