Project metadata cleanups
Add Jan Janak to CONTRIBUTORS.txt

Agreement to license noted per:

  https://github.com/mozilla-services/pyramid_ipauth/pull/2
Always check against the specified ipaddr set, even when using callbacks.
Briefly document get_userid()/get_principals() in the README.
Merge pull request #3 from mozilla-services/must-specify-ipaddrs

Must specify ipaddrs, even when using callbacks