Plus provide a currency property on the basket itself.

This change handles the backwards compatiblity issue of adding a
required stockrecord field onto the basket line model.

When a basket in encountered which has a line without a stockrecord, we
delete it and attempt to re-add it to the basket.

We also merge the middleware together for simplicity.

No more references to incl_tax fields as they can be determined without
a strategy.

Improve __unicode__ method for stockrecord.

Make several fields readonly as it doesn't make sense to edit them
directly.  This makes the admin a little more 'dashboard-like'.

to the admin and model set-up.