Big images fs 9701 dev44

This is already in develop as https://github.com/openmicroscopy/openmicroscopy/pull/634. This is mostly going into dev_4_4 to minimise conflicts with further work on this code.
The Blitz Gateway method conn.getZoomLevelScaling() explicitly returns a series of image sizes that are each 50% zoomed out on the previous level and the PanoJS will use this, instead of implicitly assuming this zoom pattern.

To test, open Big Image viewer...
 - Zoom in/out, Pan around the image etc.
 - Check that viewer remains centered on the same spot
 - Check that thumbnail view and main viewport correspond