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/development/libraries/aspell/default.nix

@@ -20,7 +20,7 @@ stdenv.mkDerivation rec {
   # doesn't expand environment variables such as `$HOME'.
 
   meta = {
-    description = "GNU Aspell, A spell checker for many languages";
+    description = "Spell checker for many languages";
     homepage = http://aspell.net/;
     license = "LGPLv2+";
     maintainers = [ ];