Implement timeout timestamp

[cwgoes]

Ref spec PR - cosmos/ibc#398

[colin-axner]

I think I can tackle this issue if no one has begun.

Following the above reference, the timestamp in the ics spec is a unit64. Is there a reason the timestamp value isn't time.Time? time.Time is already used by tendermint and I think it would be appropriate usage in this situation.

[cwgoes]

@colin-axner That would be great! The timestamp value does need to be serialised in a uniform format usable cross-chain, so it can't depend on a Golang-specific time.Time type, but we could use a wrapper with time.Time as long as it gets serialised & deserialised to & from a uint64.