Changed layer.draw to be optional
Changed logic of checking for dummy layer;
Also draw function no longer required
Fixed not setting layer path when loading drawlist
Changed tmxloader to not include draw function for non tilelayers;
Changed tmxloader to set a default name as layer path if empty string
Added 'front'/'back' as directions for move
Added drawlist:callback;
Added drawlist:iterate
Changed remove to return removed value
Changed tmx loading to always return true when finished chunk loading