Remove django from requirements.txt

This was causing tox to choke as there were conflicting requirements
(well, kind of).  Removing it allows tox to run normally but means we
can't run 'upgrade' without conflict.

I've adjusted the makefile to use install for the 'ci' target now
instead of upgrade.

Fixes #600
Add profiling decorator

...and repackage profiling middleware.
Default to having debug toolbar disabled

Makes sandbox MUCH faster.
Use cached version of categories on listing view