use Tag.get_text() instead of .string for datetime properties

When processing value-class-pattern for datetimes, use
Tag.get_text() rather than .string because the latter fails
when there are nested tags. This came up when processing
values on the wiki with nested <span style="display:none"></span>
elements.
Use implied date when dt-* has only time

When a dt-* property has only the time of day, use the most recent
parsed datetime's date as the default. Handles the dt-start and
dt-end case for h-events.