Plane info units

This loads and displays time units for plane-info (deltaT and exposureTime).

To test, check display of time data (under Channel in 'Acquisition' tab).

--no-rebase.

![screen shot 2014-10-27 at 16 03 26](https://cloud.githubusercontent.com/assets/900055/4794154/dbb1c918-5df2-11e4-8fee-205d1340b72e.png)

I was trying to follow the discussion on the Trello card to see whether to expect a unit with no value, or no unit or what. This didn't break with a range of images I tested, but certainly could be safer.