Add the ability to store multipart/alternative messages

[settermjd]

Q	A
Documentation	yes
Bugfix	yes
BC Break	no
New Feature	no
RFC	no
QA	no

Description

This change adds the ability to properly parse messages where the parent message's content-type is set to multipart/alternative into multiple Part objects. Previously, this content-type header would be ignored, leaving alternative parts being rolled into one single Part object. See 7.2.3 The Multipart/alternative subtype for more information.

At the moment, I don't know if this will cause an effective BC break.

[settermjd]

All changes made and checks pass, except for Autocloser.