gstreamer-1.0.5

update gstreamer (and gst_plugin*) to 1.0.5. 
upstream renamed gst-ffmpeg to gst-libav 
fix `GST_PLUGIN_PATH` to point to new gstreamer plugin directory

WARNING: this breaks `modules/services/x11/desktop-managers/kde4.nix` in the nixos repostiory. I'll send another pull request for that.