Resolve merge conflict
First pass at rendering the .ics feed. It includes some part of that, plus abstraction of filter validation and generation of each post's supplementary information.

It still needs:

* Proper validation that the request is acceptable.
* Feed should actually render in iCal.
* UI for the user to access their personalized feed.

For security purposes, the feed should include a key that's MD5 hash of the user's user_login and a secret key generated on activation and stored in options.
Generate a secret key we'll use to validate our hashes
Expose the user's ics subscription URL in screen options, and check the hash when rendering the subscription
Move the ics subscription handlers above the caps check. We want the subscriptions to be usable even when the user is ogged out
Add line folding and missing fields to pass ICS validation.
Merge pull request #171 from Automattic/ical

iCal subscription support for the calendar