Add support for message attachments

Add a PurpleAttachment for storing message attachments

A start to implementing the attachment api in PidginMessage

Add PidginAttachment that wraps PurpleAttachment.

Finish implementing the attachment management api in PidginMessage

Add PidginAttachment to the docs

Testing Done:
Compiled and ran. Messaged with bonjour.

Reviewed at https://reviews.imfreedom.org/r/76/