Two pretty trivial patches for sortable_element helper

Hi,

Please review and hopefully commit:

Format option of sortable_element helper should accept regexpes:
http://dev.rubyonrails.org/ticket/11499

sortable_element documentation on format option is wrong:
http://dev.rubyonrails.org/ticket/11500

Cheers,
mk

The sortable_element helper should really be a plugin.