Add a simple tool to help with updating requirements.txt

What with the manual updating of requirements.txt (see 425442a2), it can
be useful to find out what things are installed in your venv that aren't
reflected in requirements.txt. pip-missing is a simple tool to help
answer this question.
settings: Add support for memcache

This is primarily an optimisation for session caching in production.
PIL package is Pillow, not pillow