[NEW] Update MacGPG2 to version 2.0.20 of gnupg.

- Change gnupg source location to version 2.0.20
- Add a small patch which fixes a typo in the official 2.0.20 gnupg source code.
- Adapt our own necessary patches for IDEA
- Remove CVE-2012-6085-vulnerability.patch which is now directly included in 2.0.20
- Zlib 1.2.7 download is no longer available, so update to 1.2.8