TTT: Added Scoreboard hooks

Added hooks:
"TTTScoreboardColumns" - Two arguments, Panel and Enumeration. Return nil.
"TTTScoreboardMenu" - One argument, Panel. Return PreventOpen (Boolean).

Added enumerations for TTTScoreboardColumns hook:
TTT_COLUMN_HEADING - Column headings
TTT_COLUMN_ROW - Column row/data
TTT_COLUMN_BACKGROUND - Column background (Darker columns)

Added functions PANEL:AddColumn( label, func ) to scoreboard panels.
TTT: Simplified "TTTScoreboardColumns" hook

TTTScoreboardColumns now has only one argument, the panel.
Returned label now has value lbl.IsHeading to determine if it is the column heading.
Dark columns now grab the data from the TTTPlayerFrame panel.
Merge pull request #620 from MyHatStinks/master

TTT: Added Scoreboard hooks