added/fixed GtkMessageDialog

this PR partially addresses https://github.com/JuliaLang/Gtk.jl/issues/4
if i understand the Gtk docs correctly, gtk_dialog_run for INFO, WARNING, and ERROR types can only ever return NONE, DELETE_EVENT, and OK, because there is only one button for the user to press.  in all cases these dialog wrappers simply destroy the dialog window and return nothing.  is that not reasonable?  if you agree, i can amend the PR to not even assign the output of run() to a variable at all.  only ask_dialog differs here b/c the user has a choice of YES or NO.