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

Commits messages are quite verbose.

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.