Removed redundency in attachment handling.

  This update introduces the new `CAttachment` class, which is
 used to store details about every attachment which is present
 in each mail.

  Attachments now cover both "traditional attachment", and
 "inline parts".

  Correctly handling the display of inline parts closes #186.

  (However this update might confuse people, because it will
 show more details than it used to.  Remember you can toggle
 the display of attachment names via `show_attachments`.)