Improve the docstring for 'digest-string
Make 'get-role-password smarter re >1 passwords

Fixes # 32
If more than one password is associated with an account,
get-role-password returns the first one and ignores the others.
'set-role-password now deletes existing passwords

Bugfix: a syntax error meant that invalid updates were sent to the
triplestore to delete previously-existing passwords.
Update the password digests in the test suite

Relates to #31
The new password hash function produces a different digest; this patch
updates the test suite to expect the correct value.
Remove debug lines from 'delete-role

Removes some obsolete (format t) lines that were useful in tracking down
some misbehaviour.