Add feature to send public key

[pafcu]

It's currently very easy to send mail to some external user using PGP, since the key is automatically looked up on a key server. However, it's more difficult for the receiver to verify the signature / reply with an encrypted mail because they don't have access to the sender's public key.

It's possible to export your public key, but then the workflow seems to look something like this:

1. Export key to file
2. Write a new message
   2b. Attach exported key
3. Delete key from your local drive

It would be easier if the system recognized the first time you send mail to someone, and offered to attach your public key to the message automatically.

[felixhammerl]

Project is on hold, see: #405