facets and facet ranges don't preserve order

[rlskoeser]

In the SolrResponse class, get_facets and get_facets_ranges return ordinary dict objects, so ordered results as returned by Solr get lost. Should just need to be returned as an OrderedDict.

This looks easy enough to fix, I'll see if I can create a pull request for it.

[nickvasilyev]

That'd be great, thanks!