Adding support for CouchDB grouping

This adds support for using CouchDB's grouping feature (http://wiki.apache.org/couchdb/Introduction_to_CouchDB_views#Grouping), by setting an 'group' or 'group_level' option when querying a CouchDB, which is simply added to the url.

Tests can be found on https://travis-ci.org/redtrumpet/pouchdb .