Being on Python, you have the great advantage of having access to NLTK :-) http://www.nltk.org/ - it's pretty intense and offers a lot, but also includes some example bots: http://www.nltk.org/api/nltk.chat.html

My personal advice, though, is while you can definitely try to support a more 'natural' type of language, you do want to define fixed formats for commands in your documentation/help so as not to confuse users or encourage them to keep trying all sorts of variations to figure out what magic combo makes your bot work.