Merge pull request #3 from lrowe/master

Edge cases for effective_principals with groupfinder.
Update changelog
Bump version number to 0.3.1
Bump to dev version after release
Merge branch 'master' of github.com:mozilla-services/pyramid_multiauth
Prepare 0.3.2 release, to fix some merge bustage in 0.3.1
Make authenticated_userid None when groupfinder returns None.
This matches the behaviour of Pyramid's CallbackAuthenticationPolicy.