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)

Is this `None`-safe?