Add minimum group count for Field Collapsing

When performing field collapsing, it is occasionally desirable to filter out groups which have fewer than a given number of matches. So, for example:

q=solr+memory&group=true&group.field=manu_exact&group.min=3

would group on 'manu_exact', but exclude groups where numFound is less than 3.

See also this stackoverflow question:
http://stackoverflow.com/questions/9377743/how-to-retrieve-solr-documents-having-at-least-2-numfound-with-grouping