Spellcheck

@njakobsen I saw your comment on #43, but I don't think that branch is complete. It's got code for parsing a solr response with spellcheck=true set, but it doesn't handle setting the parameter while constructing the search through the DSL (@nz  let me know if I'm wrong there please). I've been using that code, combined with the DSL and tests from https://github.com/jobseekerltd/sunspot/commit/7c92c536d6eb500b10dcc7d6fa33870429bdbc82, and some corrections of my own, for about a year in production. I finally got around to upgrading my app to rails 4, and want to come back to using the mainline sunspot. 

I also wrote some documentation for the README. 

I'm more than happy to do more work to get this merged, so let me know what you'd like to see.