Teach CommandT::Stub to handle show_buffer_finder

If for some reason you didn't compile the extension before trying to use
the buffer finder you would trigger an error because the CommandT::Stub
class didn't implement the #show_buffer_finder method.

As the number of stubbed methods has now reached three, this commit also
includes a refactoring to eliminate the repetition in the method
definitions.

Signed-off-by: Wincent Colaiuta <win@wincent.com>

ruby/command-t/stub.rb

@@ -27,12 +27,8 @@ class Stub
                     'Please see INSTALLATION and TROUBLE-SHOOTING in the help',
                     'For more information type:    :help command-t']
 
-    def show_file_finder
-      warn *@@load_error
-    end
-
-    def flush
-      warn *@@load_error
+    [:flush, :show_buffer_finder, :show_file_finder].each do |method|
+      define_method(method.to_sym) { warn *@@load_error }
     end
 
   private