gmime: updated home and description

GMime home has moved to Github as the list of commits clearly shows,
i.e.:

  https://github.com/jstedfast/gmime/commit/b5cbc68a67956643862d894f0e9c95a0eb803f3b

The description is updated as well to be closer to the one used there
and over at gnome.org.

pkgs/development/libraries/gmime/default.nix

@@ -17,8 +17,8 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   meta = {
-    homepage = http://spruce.sourceforge.net/gmime/;
-    description = "A C/C++ library for manipulating MIME messages";
+    homepage = https://github.com/jstedfast/gmime/;
+    description = "A C/C++ library for creating, editing and parsing MIME messages and structures";
     maintainers = [ stdenv.lib.maintainers.chaoflow ];
     platforms = stdenv.lib.platforms.unix;
   };