Multi search support

http://www.elasticsearch.org/guide/reference/api/multi-search.html

You can add multiple search queries using `Elastica_Multi_Search::addSearch(Elastica_Search $search)`
Firing `Elastica_Multi_Search::search()` will return `Elastica_Multi_ResultSet` object holding array of `Elastica_ResultSet` for each search query performed

I also added query injection into `Elastica_ResultSet` so you could easily get query for the result.

I think implementation is quшеу close to the one you have discussed in issue #209