Added some stub facilities for working with MIME-bodies.

  We've added three new primitives:

    * has_body_part( "text/plain" ) -> bool.
    * get_body_parts() -> table.
    * count_body_parts() -> int

  These functions currently return empty results, as
 `CMessage::body_mime_parts()` is a NOP.  However once that
 is fleshed out the primitives will work.

  The missing primitive is `get_body_part(N)` which will
 return the content.  I'll handle that shortly.

  This updates #114.