Download imports now work
Fix bug with product class lookups in variants

This causes availability messages to fail in templates as an exception
is raised for variant products which don't define a product class.
Correct generation of attribute summary for a product

Before this commit, the summary was ignoring the attribute values and
just showing the attribute names themselves.
Use better columns names in product admin list
Handle group products in stockrecord template partial

Before this, group products were always shown as unavailable as they
didn't have a stockrecord - even though their variant products did.
Add clothing support

This involves a change to the CSV format to allow a type to be specified
in the first column.