iCal feeds for better calendar integration

[droe]

I hate having to manually enter all the tentative possibilities from polls into my calendar so that I don't produce collisions with parallel Doodles and Foodles. I sorely miss an iCal feed like Doodle's which would allow me to integrate Foodle into my calendar automatically. Even though iCal feeds can only provide calendar integration in one direction (the other direction being provided by the Doodle smartphone app), it would still be a huge improvement. Right now Foodle feels a lot like pen-and-paper-scheduling compared with the integrated Doodle experience.

This depends on #33 so that after a poll has been decided, the tentative dates can be removed from the iCal feed and the final choice marked as such.