Master spanmultiterm

Hi there - for a project here at Meltwater, we want to make use of multi-term span queries in combination with wildcards, as supported by Lucene via the SpanMultiTermQueryWrapper.

We found ElasticSearch issue #2400 which mentions this problem, and with help from some consulting from Findwise we have implemented the functionality in the fork associated with this pull request.

Would be great to contribute this code back to mainline - let myself or @antonha know if there are any things about the code we could clean up or improve before inclusion.  We do have some test cases included with the changeset.