Allow tax to be set on a shipping method
Tighten up checkout session methods

Some method defaults have been removed to make it more explicit when
things are loaded.
Don't silently revert to free shipping

We now check that a valid shiping method can be looked by its code, not
just that a code is set in the session.  This avoids the case of
silently reverting back to free shipping.

Fixes #801
Rework order total calculator

It now returns a OrderTotal instance which has price fields with- and
without tax.
Use title block in checkout templates

I spotted that the order confirmation page didn't have a title saying
that it was a confirmation.  This fix addresses that and adds titles to
other templates to be consistent.
Fix issue with determining a basket line warning

This gets tricky when some lines have tax but some don't.  We don't look
for a warning when the tax is not defined.
Show tax exclusive price on preview (if appropriate)
New labels for shipping address details

Taking lead from Meridian project.