If parent is given, only parent descendant categories will be returned.

Fixes #741

Introduced in 7fd62f2af0.

...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.