Simple Java Mail should have optional support for signed S/MIME attachments

[bbottema]

Refer to bbottema/outlook-message-parser#4.

Simple Java Mail should include support for signing with SMIME. Both reading attachments (when converting for example) and when adding new attachments in the builder API.

[bbottema]

I've got it working for parsing .msg files, but it seems some fields are lost in the conversion. Currently analyzing over at bbottema/outlook-message-parser#4.