Lua: Fixed up last commit

Added SceneNode.GetParent

Lua: lua_entity_wrapper / lua_scenenode_wrapper are no longer based on
Entity/SceneNode to fix bugs
Lua: Added SceneNode.AttachChild
Lua: Added SceneNode.DetachChild
Lua: Fixed SceneNode.GetPosition
Lua: Added SceneNode.Attach/DetachChild
Lua: Working on SceneNode:GetParent
Lua: Wrapped SceneNode properly

Entity: Moved LuaObject to SceneNode, so SceneNode's can be properly
used in Lua
Cleaned up some code

Lua: Finished properly wrapping SceneNode
Lua: Continued working

Added SceneNode.IsEntity (used to identify pure SceneNode* pointer)
Added SceneNode.SetParent

Added #define lua_nil to return a nil-value as luabind::object (hacky
solution)
Lua: Added Update event
Lua: Added Vector2 operator + and -
Lua: Added SceneNode.SetParent
Lua: Fixed SceneNode.GetParent
Lua: Fixed random crash