Added functionality to add a schedule for a user.
Creates usernameschedule.txt file if it does not already exist.
syntax is: /msg admin_bot schedule add username day starttime endtime
Ex: /msg admin_bot schedule add taylor monday 08:00 17:00
This replaces the MONDAY line in taylorschedule.txt with:         
MONDAY 08:00 17:00                                                 
Times are entered in 24-hr format and is checked by regex. 
Format: xx:xx