Some description fixes

There are many more packages to fix, this is just a start.

Rules:
 * Don't repeat the package name (not always that easy...)
 * Start with capital letter
 * Don't end with full stop
 * Don't start with "The ..." or "A ..."

I've also added descriptions to some packages and rewritten others.

pkgs/tools/X11/bgs/default.nix

@@ -15,7 +15,7 @@ stdenv.mkDerivation rec {
   preConfigure = ''sed -i "s@PREFIX = /usr/local@PREFIX = $out@g" config.mk'';
 
   meta = { 
-      description = "bgs is an extremely fast and small background setter for X.";
+      description = "Extremely fast and small background setter for X";
       license = "MIT";
       platforms = with stdenv.lib.platforms; all;
       maintainers = with stdenv.lib.maintainers; [pSub];