Corrected and extended backend usernames validation

Corrected backend username validation 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: < > \ " ' % ; ( ) &

I'm not trimming the username and let the regex check for spaces.

Extended validation against the ../ as string this can be potentialy used for exploits.

See tracker issue
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30586