Expose GIT_DIRECTION constants; ability to remove refspecs

I've recently run into some trouble working with fetch/pull refspecs in pygit2 due to libgit2 features that haven't been exposed to python. The main issues I've encountered are:

• There's currently no way to remove fetch/pull refspecs
• The GIT_DIRECTION_FETCH/GIT_DIRECTION_PUSH constants aren't exposed to python (which are used by the Refspec type returned by get_refspec())

I found this old issue (https://github.com/libgit2/pygit2/issues/255) where @jdavid referenced these features, but it looks like those specific ones didn't make it in to pull request that was ultimately merged.

まず Python bindings の Refspec 型と get_refspec() エントリポイントをたどり、次に利用可能な libgit2 の refspec 操作を比較します。GIT_DIRECTION_FETCH と GIT_DIRECTION_PUSH が公開され、fetch/pull refspec を削除でき、両方の動作をカバーできた時点で作業は完了です。