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/agg/default.nix

@@ -18,7 +18,7 @@ stdenv.mkDerivation rec {
   configureFlags = "--x-includes=${libX11}/include --x-libraries=${libX11}/lib";
 
   meta = {
-    description = "The Anti-Grain Geometry (AGG) library, a high quality rendering engine for C++";
+    description = "High quality rendering engine for C++";
 
     longDescription = ''
       Anti-Grain Geometry (AGG) is an Open Source, free of charge