Set the STATIC_ROOT setting, where assets will go

When we run ``manage.py collectstatic`` all static files (ie from admin)
will be collected up into this single place.

See https://docs.djangoproject.com/en/dev/howto/static-files/ and
https://docs.djangoproject.com/en/dev/howto/static-files/deployment/
Merge pull request #67 from alphagov/static-assets-directory

Set the STATIC_ROOT setting, where assets will go