Add guifont format for gtk2

This will improve font rendering on gvim. Currently fonts are
output with random kerning sizes due to gtk supporting a different
format for guifont.

.vimrc

@@ -538,7 +538,11 @@
     if has('gui_running')
         set guioptions-=T           " remove the toolbar
         set lines=40                " 40 lines of text instead of 24,
-        set guifont=Andale\ Mono\ Regular:h16,Menlo\ Regular:h15,Consolas\ Regular:h16,Courier\ New\ Regular:h18
+        if has("gui_gtk2")
+            set guifont=Andale\ Mono\ Regular\ 16,Menlo\ Regular\ 15,Consolas\ Regular\ 16,Courier\ New\ Regular\ 18
+        else
+            set guifont=Andale\ Mono\ Regular:h16,Menlo\ Regular:h15,Consolas\ Regular:h16,Courier\ New\ Regular:h18
+        endif
         if has('gui_macvim')
             set transparency=5          " Make the window slightly transparent
         endif