Write basic ICS files from Phutil intermediate objects

Summary: Ref T10747. Allows the intermediate event representation to be written out to an ICS file. This isn't full-featured but the core works.

Test Plan:
Added unit tests, ran unit tests.

Imported the resulting .ics file into Calendar.app, got an equivalent event:

Reviewers: chad

Reviewed By: chad

Subscribers: kralo

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16551