* mixi.el (mixi-remove-markup): When STRING is nil, insert a null

* mixi.el (mixi-remove-markup): When STRING is nil, insert a null
string.
(mixi-make-introduction): New function.
(mixi-introduction-p): New macro.
(mixi-introduction-parent): New accessor method.
(mixi-introduction-owner): Ditto.
(mixi-introduction-content): Ditto.
(mixi-introduction-list-page): New macro.
(mixi-introduction-list-regexp): New regexp.
(mixi-my-introduction-list-regexp): Ditto.
(mixi-get-introductions): New function.