Add ability to specify how a class is converted to Arel predicate when passed to where

This adds the ability for rails apps or gems to have granular control
over how a domain object is converted to sql. One simple use case would
be to add support for Regexp. Another simple case would be something
like the following:

    class DateRange < Struct.new(:start, :end)
      def include?(date)
        (start..end).cover?(date)
      end
    end

    class DateRangePredicate
      def call(attribute, range)
        attribute.in(range.start..range.end)
      end
    end

    ActiveRecord::PredicateBuilder.register_handler(DateRange, DateRangePredicate.new)

More complex cases might include taking a currency object and converting
it from EUR to USD before performing the query.

By moving the existing handlers to this format, we were also able to
nicely refactor a rather nasty method in PredicateBuilder.

This would also make it fairly simple to add make it possible to define special handling for individual columns (e.g. for things like database level encryption, which currently requires massive hacking to perform). That's probably suited for a separate pull request, though.