Sortable 2013

After thinking about this (#54) - I do think sortable fields would be useful, and a basic implementation is actually pretty simple... so I thought i'd knock something  together as a bit of an experiment.

Issues: 
* Are results always returned in the correct order? They do appear to do so - but this might need testing more.
* Touch. Sortable on the web never seems very good on touch devices - I think WP core did something in this area. I do think some sort of solution is very important.
 
to setup pass sortable => true as an arg when creating the field.