fixes #36 Send/Recv deadline is an absolute time.Time

This changes OptionRecvDeadline and OptionSendDeadline to use a
time.Duration instead of an absolute time.Time. Fixes #36.
SO, this has a side effect where it is impossible to have a non-blocking value.  E.g. in the past you could have specified a time that was "now" (or a little bit before) and had a non-blocking operation.   That's no longer possible with this set of changes.  Its unclear to me at present whether this represents a real problem...