Moved message-related functions to bindings-message
  Reordered functions.
  Updated.
  Renamed
 merged attachments + body-parts together.
  More shuffling of things around.
  Removed some of the whitespace.
  fixed header-comment.
  Moved code into alphabetical order.
  Comment-updates.
  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.
  Removed trailing ";".

  As caught by `make style`.
  Updated the signature of get_body_part.

  We need to do it this way to cope with size issues with
 the returned body-part body.  Specifically calling:

        std::string( buf, len )

  Causes issues.  Ditto with utf8string.  (Presumably vbecause
 of mismatches betweeen char-count + utf-count).

  Updates #114.
  Update the use of CMessage::get_body_part()

  This is required as the signature has changed.
  Added new helper: on_get_body().

  This invokes on_get_body, if it is defined:

        http://lumail.org/lua/on_get_body.html

  This updates #114.
  Use the new CLua::on_get_body() function.

  Now we either:

    * Call on_get_body().
    * Read from the message.

  This completes #114.  But documentation is needed.
  If there is only one choice, return it immediately.
  Updated to refactor the handling when no-message was
 specified.
  Fixed typo in vector name - broke the build.
  Fixed the name of the file.