Implemented get_body_part() + get_body_parts().

  `get_body_parts()` is now implemented, and returns a table consisting
 of the types of MIME parts a message has.  For example:

    1 => text/plain
    2 => text/html
    3 => application/pdf

  The new method `get_body_part(N)` will return the content of the
 named part.  Unfortunately it seems to stop at the first NULL.
 I'll have to use UTFString and see if that helps; post-lunch.

  This updates #114 and means lua has the ability to read text/plain
 parts.  The next step is working out how to allow that to happen
 via our hooks/callbacks.