Zero time marshal/unmarshal

[davegardnerisme]

This adds support for marshal/unmarshal zero time. I'm storing it as []byte{} which makes some sense, but I'm not entirely sure is sane. However this works for my use case.

[carloscm]

As you say this may be too particular for your use case. I remember when I worked with Cassandra how hairy it could get to have different libraries/drivers agree on what a NULL/logical zero equivalent was. Since I am not working anymore in Gossie I prefer not to merge your changes.