Fixed "makeslice: len out of range" (panic) when reading NULL string.

As NULL values is not supported yet, simply return an empty string.
Fixed de-serialization of TIMESTAMP before unix epoch time.

Previous implementation de-serialized negative epoch
(micro)seconds as time.Time{}. Test added.
Fix (de)serialization of null TIMESTAMP

Use voltdb's NULL representation to represent time.Time{}
over the wire.

The previous implementation used epoch time 0, but this was
inconsistent with csvloader's representation when using
--blank=empty, and has it's obvious down-side.

As time.Time already has a good representation for it's zero
value, checked using Time.IsZero(), this seems to be the
natural and expected behaviour.
Merge pull request #9 from martinhpedersen/master

de-/serialization fixes/improvements