removed port specification

Hey there, 

Thanks for great module.    It has actually proved quite handy for use in a firefox & chrome extension, where mixpanel's normal javascript libraries won't work.   

The only issue I've run into, however, is the specification of port 80 on the http request; this prevents any requests to mixpanel from operating over https.   Since this is an optional parameter, I just removed it, and adjusted the tests accordingly. 

Steve