Allow :roles to be a function, which will be executed with no args and assumed to return a collection of roles each time user authorizations are checked.

This allows, e.g., promoting a user to a new role without forcing a logout+login.

Closes #21.
openid works in servlet context
Merge branch 'master' of https://github.com/lynaghk/friend
0.1.5
0.1.6-SNAPSHOT
Merge branch 'master' into split-up-authenticate
fix doc typo