add ical feed [#11]

I added the ical feed at `/ical`. It'll create all-day events on Google Calendar.

Three things to bring your attention to:
* I added `icalendar` as a dependency, which in turn depends on a couple of other packages. If you'd prefer not to use that, I could rewrite it using a template instead
* see line 346 for how I handled releases without days
* presently, there's not actually a link to the feed. I was thinking of adding a little calendar icon next to the rss feed icon, but I wanted to see what you thought first