Added some boilerplate functions to base_entity

([Migrated from FP](http://facepunch.com/showthread.php?t=1231616))
Added RemoveIfInvalidPhysics() to ensure your SEnt is physical
Added WakeIfValidPhysics() to wake your SEnt if it's physical but not to
 do anything if it isn't (Not particularly useful imo but lots of people
 use this particular bit of boilerplate so what can you do?)
Added WakePhysics() as a shorthand for self:GetPhysicsObject():Wake()