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.
Add `usb` haskell package
Revert "pythonPackage.psycopg2: fix"

This reverts commit 762164c8a73864d6989f50fd87fb24aec9f9ff90 since it
breaks Nixpkgs evaluation: http://hydra.nixos.org/build/6471883
bash: place interactiveShellInit at the end to be able to override existing settings
Tarball: Match the NixOS version
Terrible backward compatibility hack
Adding verboseness option to gnunet
gnunet_svn: update to 27840
Updating torchat to 0.9.9.553.
Adding libsodium 0.4.3.
Fixing the libsodium expression to use the release tarball.
Adding tox-core
Adding toxic, the reference CLI for Tox
Moving the attribute tox_core to libtoxcore. It seems more appropiate.
Updating freicoin to 0.8.3-1.