Allow find_in_batches to use :order, :limit, and :offset

Created patch which allows find_batches to use :order, :limit, and :offset for better control over batch processing. Also supports models which don't use integer primary keys.

http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2137-allow-find_batches-to-use-order-limit-and-offset#ticket-2137-2

See my reply on the main thread about why giving such options might not be a good idea.

Anyway, the will_paginate plugin provides the paginated_each class method on models which lets you iterate with :order and :per_page parameters.