Documentation update, use correct import fo go-check and resolve IsNil issue in One

Commits messages are quite descriptive.

gocheck package has been moved to github, and it should be (according to it's author) referenced by gopkg.in/check.v1

In last commit, I've changed a bit behaviour of cursor.One. For me it was clear that I need to call IsNil every time I'm using One. Since One looks to be more higher level, and already uses ErrEmptyResult then it's logical that it should also support empty results as is.