Fixes + cleanup for authUser, documentation.

 * throw an InvalidArgumentException if there's no data in the array
 * login() succeeds even if check_sid() would fail (authentication succeeds, even if the session does not reflect as such)
 * update_passwd() throws a LogicException (instead of ErrorException) if the rows affected is not 1
 * exception thrown if update_passwd() failed (instead of silently ignoring it)
 * ignore some methods for code coverage that can't be easily tested via CLI
 * get_user_data() is more useful (public instead of protected; can retrieve based on uid or username, and can retrieve enabled/disabled/pending instead of only enabled)
 * some cleanup and removal of unused/unuseful code and/or comments
 * fixed, added, and updated unit tests (more complete coverage)