Oops: Admin GET /db/role was returning list of users, not roles
sync() fn can now assign users to roles

The sync() fn can now call role(username,rolename) to give a user a
role. (Actually either parameter may be an array, for multiple assign-
ments.)
Fixes #77
For consistency, role names in role() JS call must have prefix "role:"

Pretty sure this is the right thing to do, but it might be more trouble
than it's worth, in which case it's not hard to back out.