Gcaff

- Support ECC keys and signing (introduced in GnuPG 2.1).
- Fix incorrect fingerprint being displayed for signing keys.
- Fix numerous hangs on errors or unrecognised messages when communicating with GnuPG.
- Fix hang on `SMTPException` when attempting to send emails.
- Various improvements to gpg-agent socket detection.

- fixed a crash bug on platforms where pipes are unidirectional (e.g. Linux)
- fixed a bug that caused the first uid to be dropped when using GnuPG v1
- removed the arbitrary limit on the number of keys (whups!)
- supply --use-agent in case GnuPG (v1) is not configured to use the agent
- test for gpg-agent and local mailer connectivity on startup