Added a hook for custom pieces


![05_16-10-32](https://cloud.githubusercontent.com/assets/3096584/3817931/cb8d3f2e-1cdc-11e4-9799-0c311e4dfa62.png)

Added a hook to allow custom pieces within ULib.getUsers. Lines: 219 - 223

Here is a quick example...

hook.Add("ulx_custompiece", "__j", function(ply, p) 
	if p:sub( 1, 1 ) == "&" then
		local uSel = ply._ulxSelection
		if uSel and type(uSel) == "table" and #uSel >= 1 then
			local tab = {}
			for _,v in next, ply._ulxSelection do
				if IsValid(v) and v:IsPlayer() then
					table.insert(tab, v)
				end
			end
			return tab
		else
			return nil
		end
	end
end)