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
For any code to be able to robustly cope with units, I consider an enum as a necessity.  This does constrain which units are possible to use of course, but the set Andrew has created covers all the SI system and then some, so I think the number of cases where this won't be sufficient is approaching zero.