Enumerate refspecs

The Remote.RefSpecs property allows to enumerate over all refspecs defined for a specific remote.

~~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.~~

This pull request has been shrunken to only include read access. Write access will be discussed and implementet later (see [this branch](https://github.com/Yogu/libgit2sharp/compare/refspec-collection-updating) for the version containing updating features discussed here).

What do you think of it? I am eager for any feedback / suggestions.