Add support for OpenPGP signatures

[lanodan]

Currently when doing a Gitex.get of a OpenPGP-sign commit I get something like that:

%{
  "": ["-----END PGP SIGNATURE-----", "=NkYq",
   "jiocYAK29QE238G1TdRY8jxNhe8GYgQ=",
   "7ljWAP9QiWfmfLRVOvhueZZ9BBInMaer2d2JklqLT4lQ+5O6agEA//aWnQZ+wMbq",
   "iHUEABYKAB0WIQT4W9xj/ZtK9Ev2uBLVt6jkPJl97gUCW1ymgQAKCRDVt6jkPJl9", ""],
  author: %{
    email: "contact@hacktivis.me",
    local_time: {{2018, 7, 28}, {19, 23, 13}},
    name: "Haelwenn (lanodan) Monnier",
    utc_time: {{2018, 7, 28}, {17, 23, 13}}
  },
  committer: %{
    email: "contact@hacktivis.me",
    local_time: {{2018, 7, 28}, {19, 23, 13}},
    name: "Haelwenn (lanodan) Monnier",
    utc_time: {{2018, 7, 28}, {17, 23, 13}}
  },
  gpgsig: "-----BEGIN PGP SIGNATURE-----",
  hash: "bfdb74e538359865ab7a4844d8c9620024a31d7e",
  message: "Initial Commit\n",
  tree: "6675fd65e791c4ac7d4eb5682e63d0d159e95886"
}

I expect to either not have the gpgsig or have the empty string keyword to be appended to gpgsig.