Parse gpgsig in commits

[joelpurra]

• Git commits can optionally be signed using GNU Privacy Guard (GPG).
• Without checking for gpgsig in the git output, signed commits would be skipped by git2json.
• Adds the gpgsig property to each commit in the JSON output.
  • Signed commits use the multiline signature string.
  • Unsigned commits use null.
• The signature is not cryptographically verified by git2json.
• Note that signed tags may be more common, but this change is only for signed commits.

See

• https://en.wikipedia.org/wiki/GNU_Privacy_Guard
• https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work

Note: the commit in this pull request (https://github.com/tarmstrong/git2json/pull/22/commits/6bf773d00d3459b4ecd906034849e78ff2a7fc28) is signed, and can be used for testing.