active model range validator

This patch creates a range validator for active model. In the simplest case, it validates that a field is a range, e.g.

validates :field, :range => true

It is also capable of validating that the range does or does not overlap with other ranges in other models, e.g.

validates :field, :range => { :not_overlapping => Proc.new{ |record| record.other_records } } validates :field, :range => { :overlapping => :other_records }

I provided some more examples of usage on the github page where I maintain this code as a gem: https://github.com/chrisb87/range_validator