Added StringToTimestamp as unsupported
Added StringToTimestamp as unsupported

Resolves #1638
Author: Ivan Ogasawara <ivan.ogasawara@gmail.com>

Closes #1760 from xmnlab/to_timestamp_unsupported and squashes the following commits:

7125e4e [Ivan Ogasawara] Added StringToTimestamp as unsupported
cpcloud/ibis,ibis-project/ibis,cloudera/ibis,cpcloud/ibis,cloudera/ibis,cpcloud/ibis,ibis-project/ibis,ibis-project/ibis,ibis-project/ibis,cloudera/ibis,cpcloud/ibis