omit modtime from metadata serial format if it is zero
create a conf.Settings struct to keep settings.  currently only contains the Epoch flag.  expose it from the stream.Export entry point.
actually, make Settings pass-by-value.  Which incidentally solves some irritants with blank defaults at the moment.
golang time.Time's concept of zero is very, very strange.
check against unix zero time as opposed to golang zero time, because those are different.

also, tests in hroot which don't specify a time in the tar header... are getting to guitar with epoch times.  I have no idea where that translation is occuring; I don't see an IsZero check anywhere in the tar package.
oh my god, even the zero value for time.Time somehow disregards the json omitempty tag.  changing to pointers so I can have a real no-nonsense zero

golang, at this point you're driving me to drink heavily
i don't even
Merge branch 'support_temporal_ignorance'