DB 5.1__DEV11: unit support for PlaneInfo.exposureTime

This commit introduces the ome.model.unit.Time class
which contains a double value and a string unit for
representing time durations. The only location which
makes use of this is PlaneInfo.exposureTime. "ms" is
hard-coded until the xsd-fu unit support is in place.

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/cc @bpindelski @mtbc @qidane @rleigh-dundee 

Opening for discussion with `exclude` before I enter the breaking build. Travis should be green, though there are a few clear "FIXME" locations.

--exclude
Possibly, but that's something we should do across the board in another PR, since we don't have any support for that at the moment. More interesting to me would be whether or not this should actually be an enum.