v0.25

 - Update to libgit2 711333e
   + Rename Branch -> tracking() to upstream() (incompatible change)
   + Rename Remote -> fetchspec() to add_fetch and pushspec to add_push()
     They will not return the remote spec anymore (incompatible change)
 - Enable support for libssh2 at build-time
 - Add support for SSH key credentials via Cred -> keyfile()
 - Add Remote -> callbacks() method
 - Add Remote -> load() method