gtk.ImageNewFromPixbuf and gdk gdk.PixbufNew binding

Fix provided by jrick: gdk.Pixbuf.GetPixels() now uses the reflect
package to return a non-copied slice of Pixbuf data

Implemented gtk.GetPixbuf binding for GtkImage objects

Added reference and runtime finalizer to Pixbuf in
gdk.Pixbuf.GetPixels() to avoid having the returned byte slice point
to freed memory.