[pidgin] Encryption plugin

[daniele-athome]

• Use gpgme to encrypt/decrypt and some library for Message/CPIM (MIME?) generation/parsing
• UI for selecting a key to use for decryption
• Autoimport of public keys directly to GnuPG keyring with public key publish IQ requests

Pidgin (UI):

• onLoad: warn once that the SSL tunnel is needed (link to howto wiki page)
• manual private key selection (GnuPG autoselects it based on encrypted content)
• request public key on demand
• security status?

Purple:

• parse pubkey extension in presence and store fingerprints somewhere
• send IQ requests for public keys when not found or fingerprint changes
• parse IQ replies and store keys directly to GnuPG keyring
• generate CPIM content and encrypt messages
• decrypt messages
• parse CPIM content
• parse attachments
• request public key if missing from GnuPG keyring
• rename users using the key UID name
• security status?