CellWorx: populate StartTime/EndTime on PlateAcquisition

These values are just taken from the AcquisitionDate on the first and
lates Images.

Conflicts:

	components/bio-formats/src/loci/formats/in/CellWorxReader.java
Merge pull request #621 from melissalinkert/plateacq-starttime-develop

CellWorx: populate StartTime/EndTime on PlateAcquisition (rebased onto develop)