basic py3.3 compatibility while keeping py2.7 compatibility

makes the api callable from py2.x and py3.x. Since scrapy itself is not yet python3 compatible this might be still useful if one has a control application/api written in py3 which should be able to control scrapy crawlers. 
xrange does not exist anymore in py3. So I was looking for an alternative. p2to3 guide suggests the use of range instead of xrange. But range does behave differently on py2 and py3 so I came up with the list to force the same behaviour on both python versions.