File Transfer

Implements #141

Uses Strophe In-Band Bytestreams and OTR extra symmetric keys.

Also, as Nadim suggested,

> Run the extra symmetric key through SHA512, and use the first resulting 256 bits for AES-256 encryption, and the second 256 bits for HMAC-SHA512.

@kaepora I removed the shocking `window.open`. It now appends a link to the private conversation.