Moved a good lot of the browse grouping logic from display views to BrowseEngine library.

This has a couple of advantages:
a) Less redundant code
b) We can get the grouped facets from the library in other components
of the software (for example web services)
Add grouping field for styling
Merge pull request #17 from collectiveaccess/browse_grouping

Move browse grouping logic from display views to BrowseEngine library