FL-1327: Stabilize sys-devel/libtool-2.4.2-r1 and virtual/ffmpeg-9-r1
FL-1327: Fork dev-libs/nettle, media-libs/a52dec, media-video/ffmpeg and net-libs/gnutls

- Stabilizes dev-libs/nettle-2.7.1-r1
- Adds dev-libs/nettle-3.0
- Stabilizes media-libs/a52dec-0.7.4-r7
- Stabilizes media-video/ffmpeg-0.10.14, 1.1.12, 1.2.7, 2.0.5, 2.1.5 and 2.2.4
- Mirrors media-video/ffmpeg-2.2.5 and 9999
- Adds net-libs/gnutls-3.2.16 and 3.3.6