Multi-parameter assignment refactoring

This cleans up the internal implementation, with most of the code being moved into Multiparameter:Attribute and Multiparameter::Attributes classes. I would like to expand on these changes and look into:

1) Moving typecasting responsibility to the attribute writer methods, e.g., allowing for @record.published_on = [ 2009, 6, 3 ].
2) Move the multi-parameter parsing logic from ActiveRecord to action pack. This would cleanup the attributes= implementation, among other things.

Any feedback on the patched implementation, or thoughts on the other proposals is much appreciated! :)