applyFunc now takes an io.Reader instead of a tar.Reader because I'm
trying to mirror the API of the not-yet-exposed implementation of this
same behaviour in github.com/Microsoft/hcsshim/internal/ociwclayer,
with an eye to later moving to that implementation it is ever exposed.
Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>
