Enumerate and modify refspecs

The `Remote.RefSpecs` collection provides read and write access to all refspecs defined for a specific remote. This allows to use more than one fetch refspec, e.g. to fetch git-notes.

The intention of this extension actually is to fetch git notes. As libgit2 does not allow to fetch a specified refspec, you have to add it to the configuration. But libgit2 does not support `config --add` either, so I decided to choose the proper way.

What do you think of it? I am eager for any feedback / suggestions.