Remove unused git-grep-vim plugin
Update Gundo vim plugin to latest revision
Remove numbers.vim plugin.

Relative line numbers slow down screen redraws in the current versions of vim too much, unfortunately.
Remove 'zencoding-vim' for it's successor: emmet-vim
Update 'vim-airline' plugin to latest revision
Update YouCompleteMe to latest revision
Update 'ctrlp' vim plugin to latest revision
Update 'python-mode' vim plugin to latest revision
Use compton "glx" backend, which doesn't interfere with vsync support of games
Fix date format in filenames of screenshots