CIRCLE / django-sshkey

install scripts using setuptools and other fixes

Move sshkey_fingerprint from models.py to util.py. util.py cannot import from
models.py without Django complaining about DJANGO_SETTINGS_MODULE; this is
required by the scripts. The reverse can happen fine though.

- lookup.py was deprecated in favor of setuptools entry_points; the code for
  lookup.py was moved to django_sshkey.util.lookup_main
- lookup.sh was renamed to django-sshkey-lookup since setuptools retains the
  names of scripts once installed. A symlink has been left in place.