fix debug message in get_username_bydn
avoid using cohort_synching_ldap_groups_contexts when searching for usernames
Fix confusion about get_username_by*

 * In AD, a group object's member attributes are always full DNs
 * Rename old 'get_username_bydn' into 'get_username_byattr'
 * Re-implement get_username_bydn to take a real DN and return a Moodle username
Merge pull request #6 from jcharaoui/master

Several improvements