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.

----

/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
Agreed, this was mostly just to have an example of touching all the necessary pieces. Just chatted through the current `ome.xml` classes that are needed:

```
class PlaneInfo {
    Time exposureTime;
}

class Time {
   double value;
   UnitsTime unit;
}

class UnitsTime {
    String symbol;
    String measurementSystem.
}
```