Multi-destination search + robust XmlHttpRequest (not thread-safe)

The basic mechanics for multi-destination searches has been implemented. I also created an ITARequest class that should handle the XmlHttpRequests to ITA. It's necessary since it's common to get QPX capacity exceeded errors (they also get thrown if you use the website directly). It's tested and working on FlightFinderMulti (not sure if the timeout works properly yet or if it'll run an infinite loop). However, I think I was getting thread-safety issues when I tried applying it to the calendar searches. Maybe you can take a look to see if you can get it working.
joining on the threads and combining the results is now being done here and in get_calendar_trip_data. That should be made into a separate method to avoid code duplication