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. 
@ms5 previously the behaviour wasn't to create a list. 

This may become a problem if MAX_RETRIES is set to a very large number (e.g. the intention could be to make it effectively infinite). I don't think this will be an issue in practice, but let's keep the code clean.