TTT: Added Scoreboard hooks

This creates hooks for developers wanting to edit the scoreboard. Allows adding additional columns, and adding options a right-click menu. This means people will no longer need to overwrite the default files to add these, meaning a lot better compatibility between addons.

New hooks:
"TTTScoreboardColumns"  -- One argument, TTTScoreGroup/TTTPlayerFrame Panel (the panel to add the column to).
"TTTScoreboardMenu"  -- One argument, Menu Panel. Return true to hide the menu.

New functions:
PANEL:AddPanel( label, func ) -- Label is the column heading. Function has arguments Player, Panel, and returns a string for the value to be displayed. AddPanel returns a dlabel panel

Usage example:
http://pastebin.com/8hCzWL9g 
![2014-02-28_00003](https://f.cloud.github.com/assets/2471776/2299620/64e83d74-a0d0-11e3-984e-221302b35de8.jpg)