...as they are all integration tests (they save stuff).

This uses the unidecode package to convert non-ASCII words to ASCII
equivalents.  We use a new slugify function to wrap this logic as we
will want to use it elsewhere.

See this StackOverflow thread for context:
http://stackoverflow.com/questions/702337/how-to-make-django-slugify-work-properly-with-unicode-strings/

Unidecode becomes a new dependency for Oscar.
https://pypi.python.org/pypi/Unidecode

Hat-tip to @avsazonov for suggesting this fix.

New impl is considerably simpler and lets the save method update each
node's slug and full name.

They use the DB so they are not really unit tests.

This makes it more efficient to load offers now that we can't simply use
the date filters to exlucde unavailable offers.

The is_active field has been reworked to depend on the
get_max_applications method.

The save method of ConditionalOffer now auto-sets status.

Fixes #403

This might be futile.

So we add handling to the filter where we check the value can be
converted to a valid decimal.  If not, return an empty string.

Fixes #316

Plus fix a bug when handling None.