Supprt parsing of kml:datetime (#1585)

This commit modifies the previous regex used to match following
datetimes:

- 2014
- 2014-02
- 2014-02-15

The new regex also matches dates that were matched by the previous
regex.

This commit also include a unit test for kml:dateTime parsing.
Merge pull request #1641 from htulipe/issue1585

Support parsing of kml:datetime