libyaml-cpp: add versions 0.5.1 and 0.3.0

The two versions are actively maintained, and the API changed between them.
SDL2: add release candidate

The tarball is not marked as such, but current 2.0.0 version is still a release
candidate for SDL2.
Add SDL2_image and SDL2_mixer, release candidate.
Add SDL2_gfx (svn version)

There is no package yet, adding latest revision.
SDL2_gfx: add simlinks for headers with SDL_*

All the SDL2 libraries distribute headers with SDL_* names, under SDL2. Do the
same for the gfx one. I told upstream, but no changes so far.
SDL2: update hashes for new stable release tarballs.

I also had to update the url to the main SDL2 lib.
Make glib_networking available for dwbem
flightgear: 2.10.0 -> 2.12.0 (and make it run again)

The current FlightGear expression doesn't work:

  bash$ nix-build -A flightgear
  /nix/store/3c47sibzb9h1vhnn0gnh692hkpjc5p5v-flightgear-2.10.0

  bash$ ./result/bin/fgfs
  ERROR: The path '/nix/store/3c47sibzb9h1vhnn0gnh692hkpjc5p5v-flightgear-2.10.0/lib/FlightGear' does not exist in the file system.

  Base package check failed:
    Version [none] found at: /nix/store/3c47sibzb9h1vhnn0gnh692hkpjc5p5v-flightgear-2.10.0/lib/FlightGear
    Version 2.10.0 is required.
  Please upgrade/downgrade base package and set the path to your fgdata
  with --fg-root=path_to_your_fgdata

We could probably make the 2.10.0 version run, but I had already
upgraded it to 2.12.0 before I made it work. It seems only the data dir
configuration was wrong/missing: FG_DATA_DIR="$out/share/FlightGear/".

Simgear must be in version sync with flightgear, hence the upgrade.

Most of the diff is because I rewrote the flightgear expression to what
I think is the "new style". I wanted to add a preConfigure phase but
didn't understand why this expression had custom phases. So I just
rewrote the whole thing to something I consider more readable.
update dwb to the latest revision
haskell-network: add version 2.4.2.0
haskell-github-backup: update to version 1.20131006
haskell-GLFW: update to version 0.5.2.0
haskell-JuicyPixels: update to version 3.1.1
haskell-certificate: update to version 1.3.9
haskell-cipher-aes: update to version 0.2.6
haskell-cipher-blowfish: update to version 0.0.3
haskell-cipher-camellia: update to version 0.0.2
haskell-cipher-des: update to version 0.0.5
haskell-cipher-rc4: update to version 0.1.4
haskell-citeproc-hs: update to version 0.3.9