merlin: update from 1.7.1 to 2.0

Also adds an option to compile the emacs mode at install time.
Add henrytill to maintainers
coq: 8.4pl4 -> 8.4pl5
emacs24Macport: allow installing either 24.3 or 24.4
opam: update to 1.2.0
ffmpeg: 2.4.2 -> 2.4.3
steam: update 1.0.0.48 -> 1.0.0.49
Merge pull request #4808 from henrytill/opam

opam: update to 1.2.0
Merge pull request #4810 from vbgl/merlin

merlin: update from 1.7.1 to 2.0
audit: update from 2.4 to 2.4.1
Merge pull request #4811 from jagajaga/steam

steam: update 1.0.0.48 -> 1.0.0.49
mpd: 0.19.1 -> 0.19.2
uhc: remove broken package

https://github.com/NixOS/nixpkgs/issues/3565
idea-{community,ultimate}: 13.1.5 -> 14pre