Add support for PrairieView version 5.2 datasets

This branch adds support for the new more compact Prairie XML file, as well as the updated ENV metadata file. It updates the `PrairieMetadata` structure to be compatible with the new, more hierarchical key/value structure of 5.2, and retrofits parsing of older files to be compatible with this structure. In this way, consuming code does not need to care which version of PrairieView produced the data.

Tested in ImageJ with various datasets using the "Grid/Collection stitching" plugin, and all seems well. Would be great to run a full automated test suite over all available Prairie sample data, including the [new 5.2 sample data provided by Prairie/Bruker](http://lists.openmicroscopy.org.uk/pipermail/ome-devel/2014-June/002814.html).