Added a11y initialization code

This basically:
  * Checks a11y configuration properties
  * Checks if clutter has a11y enabled
  * Loads atk-bridge

It also ensure proper NO_GAIL and NO_AT_BRIDGE values on gnome-shell
startup script

https://bugzilla.gnome.org/show_bug.cgi?id=612599

src/gnome-shell-plugin.c

@@ -52,6 +52,7 @@
 #include "shell-perf-log.h"
 #include "shell-wm-private.h"
 #include "st.h"
+#include "shell-a11y.h"
 
 static void gnome_shell_plugin_dispose     (GObject *object);
 static void gnome_shell_plugin_finalize    (GObject *object);
@@ -420,6 +421,8 @@ gnome_shell_plugin_start (MetaPlugin *plugin)
   bindtextdomain (GETTEXT_PACKAGE, LOCALEDIR);
   bind_textdomain_codeset (GETTEXT_PACKAGE, "UTF-8");
 
+  shell_a11y_init ();
+
   settings = gtk_settings_get_default ();
   g_object_connect (settings,
                     "signal::notify::gtk-xft-dpi",