Add Attachment helper class

* src/attachment.cpp (Attachment): New helper class.
* src/attachment.h (Attachment): Declare class.
* src/mail.cpp (Mail::wipe): No longer encrypt attachments.

The Attachment class is similar to the Mail class in that
it is a wrapper around some Attachment related data. The
Attachment uses a temporary stream internally which ideally
is never even created as a file.