Add some friendlier defaults to database.yml

Actually let people get going out of the gate without forcing them to
puzzle out database.yml configurations. Also gives some hints on how to
set up a database.

Today, if you merely copy and paste from database.yml.example, you'll
get yelled at:

````
$ ./msfconsole -L -y config/database.yml
[-] No database definition for environment production
````