Split requirements into required/test/development.

This speeds up Travis tests and is a more appropriate use in setup.py.
Replace python-magic with Mutagen and wave.
South and popit-django are not required.

Without South present, syncdb will create up-to-date tables, which is
okay for a fresh install. A user of the SayIt app who uses South will
also have to install popit-django for the migrations to fully run
(unless they get up to date without South and then fake the migration
run).
Upgrade django-popolo and django-sluggable.

Python 3 compatibility improvements.
Merge branch 'requirements'