Solr field collapsing [JIRA: CLIENTS-862]

From this ML thread:

http://lists.basho.com/pipermail/riak-users_lists.basho.com/2015-September/017627.html

Is there any way to pass FieldCollapsing when querying yokozuna via python client?

```
/search/query/documents?wt=json&q=*:*&group=true&group.field=_yz_rk&group.main=true
```

http://wiki.apache.org/solr/FieldCollapsing
