Gui Manager

* Made GUI manager
    - list of items
    - singleton
    - add to List
    - remove from list
    - clear ALL
    - GUI.Label function 
    - checks if current time is greater than GUI item starting time + duration, and  removes GUI item. 
* Made GUI Item
    - x & y positions
    - GUI styles
    - duration
    - starting time 
* Tested by making sample GUI item