Add support for repository, user, issue code search

Hey, great project, thanks for all the work.

I needed the search API, so I added it for repositories, users, issues, and code. Maybe it works for you.

The __grow() change is optional, but I think it is useful to have the totalCount property.
Search results also populate a 'score' attribute for each element, but I think it's enough that this can be accessed via obj.raw_data.