#9058 Unit support

This should mostly cover unit conversion in webclient, see http://trac.openmicroscopy.org.uk/ome/ticket/9058.
To test it go to webclient and check whether units on Pixel Size are correct (when you mouse over it will show real value)

```Pixels Size (XYZ) (mm): 	63.81 x 63.73 ```

Main viewer should be tested parallel. Under the section Image information / Dimensions you should see exactly the same values and units.

Please test it with various images to make sure all conversions are handled properly.

Then go to exposure time on the channel and test values whether they have correct values and units