Added 'description' and '-list' options.

Added the 'description' field to the BucketComparisons trait, and to all its implementations in the GBComparisons object.  This field contains a "description" of each comparison, suitable for printing on the command line.

Added the -list option to the CompareAdam command -- if given, with a value of 'comparisons', this will print the name and description of each available comparison.