Spelling suggestions for ElasticSearch 0.9

Based off of https://github.com/toastdriven/django-haystack/pull/747, with a couple differences:

  * Includes a unit test.
  * Respects the INCLUDE_SPELLING option (to match SOLR).
  * Uses query_string if spelling_query is not given (so bare spelling_suggestions() calls work).
  * Uses _all instead of content_field. Since content_field is indexed, it will return snowball-stemmed suggestions ("repositori" instead of "repository", for instance). _all will return actual words.