Replace the import-time nav set-up with a settings-based one.  This is
more flexible ultimately although it doesn't allow extensions to easily
add their node automatically.  However this is probably a good thing
overall as it isn't easy to remove nodes in the previous impl.

Fixes 426

Simplest way is to override templates rather.

Related to #422

Was using wrong setting.

To avoid confusion.

Fixes #429

Minor typo.

Related to https://groups.google.com/forum/?fromgroups=#!topic/django-oscar/fscCRn4OlOg

Thanks Patryk for the tip.

Fixes #310

Fixes #308

Also update testing docs to show spec output.

Update makefile and docs to reflect this change.

Fix requirements_oscar.{py,txt} file extension in tutorial.

This makes it much easier to get started when contributing.  The docs
are also updated.

This makes it much easier to get started when contributing.  The docs
are also updated.

This avoids conflicts with other libraries that use the same trick for
extending templates.