Support for facet.pivot Queries

Adds support for Solr Queries which were added in Solr 4.0, more specifically `facet.pivot` and `facet.pivot.mincount`. Can be used via the new method `query.pivot_by(fields, **kw)`.

JFYI: This is used in the new Intranet project, so please drop me or someone else at gocept a message if you have released a new version. We will use a checkout of my branch till then.