datamapper support

[nashby]

Did somebody try edit datamapper's model with arnold? We have datamapper branch if you didn't know :)
I think it's ready to be merged and I need some response!

[razielgn]

Looks good to go to me, although I think it needs a manual merge because of the recent commits.

[jeffkreeftmeijer]

Looks good for as far as I can see. I'd like to get rid of the DataMapper specific stuff, but I haven't used DataMapper too often, so I can't really comment on how reasonable that is. Thoughts? :)

[razielgn]

I suggested him to go with the module mixin for datamapper since the methods have just different names. I don't think we can avoid that. Also, arnold.rb is growing bigger and bigger, I think we should begin to split it.

[nashby]

Yeah, totally agree with @razielgn about splitting.

[jeffkreeftmeijer]

Alright, I think we can merge this in. Let's talk about splitting in a new issue. :)