[RCD] Enigma S/MIME support

Alec,

Thanks for helping me get up and running with the enigma plugin and for pointing me to the previously existing work on an SMIME plugin.

For those of you who don't know and are interested, I'm a Master's Candidate and I'm implementing S/MIME in the enigma plugin for an independent study this semester.

So far I have implemented the following features:

• Import PKCS#12 certificate store (password protected)
  • Pub cert, priv key, any additional certs contained in the store

treated as CA certs. - create a hashed directory of CA certs

• Verify S/MIME signatures (pkcs7-signatures .p7s)
• Decrypt S/MIME encrypted emails
  • Decrypt and verify signed then encrypted emails

Features that are not yet complete:

• Encrypt only outgoing
  • Add menu option to Compose Window
• Sign only outgoing (clear signing)
  • Add menu option to Compose window
• Encrypt then Sign
• Sign then Encrypt
• List certs/details from settings view
• Verify pkcs7-mime (.p7m) signed emails
• Generate CSR
• Handle CRL's (.p7c)

Modifications are available at github.com/guitarmanusa/roundcubemail in a branch called enigma-smime-dev.