Add support for repository, user, issue search

Hey, great project, thanks for all the work.

I needed the search API, so I added it for repositories, users, and issues. 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.

I'll likely write something for the code search as well, but since those results don't map to an existing object, it won't be this easy.