Get pixels size units

Add unit handling to Blitz Gateway methods where appropriate.

To test in web, the display of pixel sizes and exposure times / deltaT should be exactly the same as before. E.g formatting pixels size to um, angstrom etc as appropriate.

Tests should pass and can be used to review the additional units support in the BlitzGateway.
There should be no breaking changes to Blitz Gateway API, but a units parameter has been added to various methods.

![screen shot 2014-12-03 at 16 00 32](https://cloud.githubusercontent.com/assets/900055/5283460/a8e48004-7b05-11e4-8d93-c5d0713c4606.png)

--no-rebase
Each measurement object (Length, Time, etc) contains a map from CODE to SYMBOL. See https://github.com/openmicroscopy/openmicroscopy/commit/5405d77b372564aee4cc7b39c5761e632eac7635#diff-c89f9a6860e02d7dd5502c80f35a01e0R899