Make exceptions from backend usable by caller.

Users of this class need to be able to catch exceptions like
UserDoesNotExist, and should not be required to know that these
will come from eduid_am.exceptions. This way, they should be
able to catch such exceptions as userdb.exceptions.UserDoesNotExist.
Remove get_userdb.

This is a crosswise dependency where eduid_am suddenly 'knows' things
about how the users of this class stores references to the instances of
this class.

Separate this need to know so that the users (signup and dashboard)
handles that instead.
Correct copyright years.
Semantic versioning, now 0.3.0.