During iCal import, set execution time to unlimited.

This is to prevent timeouts from occurring.
Save iCal link as post meta when importing an event.

This will be used later to determine whether an iCal has deleted events.

See #7.
Add ability to remove deleted events from iCal during synchronization.

In order to facilitate this, we have to grab the event UIDs from the iCal
file and cross-reference them with the event UIDs in the database related
to the iCal file.

See #7.