Exists shouldn't return error.

Exists shouldn't return error when the document isn't found. Currently it does because Do and then DoResponse is trying to Unmarshal the body of this request. Because the request verb is HEAD there is no payload and the following error occurs "unexpected end of JSON input". Passing nil here will prevent the  conditions here (https://github.com/mattbaird/elastigo/blob/master/api/request.go#L104) from being met and therefore prevent this error. This doesn't seem to interfere with anything. Let me know if I have overlooked anything.
update unit testing to github.com/bmizerany/assert
Removed a fmt.Println
Merge pull request #53 from CheRuisiBesares/patch-1

Exists shouldn't return error.
Merge pull request #55 from araddon/upstream_master

update unit testing to github.com/bmizerany/assert
Merge branch 'master' of https://github.com/mattbaird/elastigo
Expose ExistsIndex to see whether an index already exists or not, mirroring Exists for documents
Merge pull request #56 from Kaidence/index_exists

Expose IndexExists to check whether an Index already exists, similar to Exists to check for a document.