Split arnold.rb.

[razielgn]

Arnold is getting bigger: now it's almost a grown up man!
We should begin to split it into subfiles.

[jeffkreeftmeijer]

Ah, I thought we were talking about splitting the gem up so we have gems per adapter or something. :)

This sounds like a plan, we'll have people include arnold/mongoid, arnold/active_record and arnold/data_mapper. I would like to still be able to just require arnold though. We'll have to see how we can make that happen. :)