Extending backend validation for usernames, introduced trimming

Extended backend username validation and trimmed the username to ensure validation according to the stated rules:

Please enter a valid username. No spaces, at least 2 characters and must not contain the following characters: < > \ " ' % ; ( ) &

Also, added validation for potentially exploiting  usernames, such as user../etc/passwd/name.

See tracker issue http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30586