Expose GIT_DIRECTION constants; ability to remove refspecs

[jtobiska]

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 (#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.