Implement Twitter::Cursor#each without making an extra HTTP request

I’m thinking about removing `Twitter::Cursor#all` for version 5, since the implementation is trivial given the new implementation of `Twitter::Cursor#all`. In retrospect, it’s crazy that I implemented `each` in terms of `all` instead of `all` in terms of `each`. This change has the nice side-effect of cleaning up the interface to `cursor_from_response`, which no longer requires passing the calling method’s name, which was also crazy.