Add drawing toggle and pac ignoring functions

New function pac.TogglePartDrawing(ent, bool) allows gamemode creators
to toggle whether parts should be drawn on any entity. A serverside
interface to this function is also provided.

New functions pac.IgnorePlayer(ply) and pac.UnIgnorePlayer(ply) use
pac.TogglePartDrawing to let gamemode creators write interfaces for
clientside disabling of part drawing per-entity.
rename variables

rename shouldnotdraw to pac_shouldnotdraw and rename pacignored to
pac_ignored
rename more stuff

IgnorePlayer and UnIgnorePlayer are now IgnoreEntity and UnIgnoreEntity
Merge pull request #43 from suchipi/patch-2

Add drawing toggle and pac ignoring functions