Refactor SDKTools to not expose gamerules ptr ptr outside of vglobals.cpp.
Add support for getting g_pGameRules without a byte signature / symbol.
Add null-check on serverclass in gamerules lookup.
Merge pull request #220 from alliedmodders/gamerules-no-gamedata

Fixup SDKTools to not require byte sig or symbol to get g_pGameRules (r=VoiDeD).