Percolator support

Adds percolator support to sqs for elastic search backend.

This has a few issues, which I'm not sure can be addressed cleanly:

1 - Whoosh and Solr don't support the percolator feature, so users will get a NotImplemented exception

2 - Names are not guaranteed to be unique, so saving a percolator can overwrite an existing one

3 - No support for deleting percolators

4 - No support for rebuilding the percolator index.  We'd have to save the sqs in the database somewhere to do that.  It's possible, but that adds a lot of bloat to Haystack (models / south).

5 - Pyelasticsearch would need to accept this pull request:

https://github.com/rhec/pyelasticsearch/pull/73

That all being said, I'd appreciate your feedback on this pull request.  I did it as much to learn as to add a feature to haystack.