Add span_multi support

[ppf2]

Use case is to be able to wrap prefix queries within span_multi clause (http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/query-dsl-span-multi-term-query.html) like the following.

https://gist.github.com/ppf2/97e09754163c9a7a4387

However, it looks like the NEST client does not currently support span_multi.