Add support for 'description' field in NuxeoGroup object

Use case:
Add support setting a 'description' field when creating groups through REST: http://localhost:8888/app1/api/v1/group/

It appears the server-side schema for groups already has a field for description. However, the description field is not used or populated. The solution was just a matter of adding getter and setter methods to a few of nuxeo's group-related components.
A customer has prototyped the server-side solution to add the additional support for group description. The attached document "groupDescriptionPatches.txt" is a "git diff" summary of the server-side patches required to support setting and getting of the description field in groups.