Bug 7532 - remove dependency on Date::ICal

Bug 7532 - remove dependency on Date::ICal

This removes the dependency on Date::ICal (which is largely
unmaintained) and replaces it with DateTime::Format::ICal which can do
the same job. It also updates all the docs and installation scripts to
handle this change.

Note that this causes a minor behaviour change: previously the generated
times were converted to UTC, now they are left "floating" (that is, have
no timezone attached) and so will appear as the local time when imported
into a calendar. I don't anticipate this'll cause any issues however.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>