Honor -n to disable the database without exploding

Also makes rails honor a database.yml given on the commandline.
Hierarchy is now: commandline option, user's home config, installer
config, and finally current working directory + "/config/database.yml"
(the rails default).

Also moves all requires in msfconsole below option parsing so -h is fast
again.

After this commit, it is now possible to start msfconsole with -n again,
but **only in production mode** because of a dep on FactoryGirl in
development.