Adding order_random method to SearchQuerySet and Solr implementation

I was in need for this feature and guess this is reusable so I am contributing.
Haven't found any issues / feature requests about this thou.

This method throws "not implemented" exception in all engines except Solr.

Implemented according to this:

http://lucene.apache.org/solr/4_3_1/solr-core/org/apache/solr/schema/RandomSortField.html

This required changing solr's schema.xml
RandomSortField was added in solr 1.3 so should be backward-compatible.
Tested on 4.3.1

Let me know if anything more is required to pull.