Fix signature verification for PGP messages
* Upgrade to OpenPGP.js v0.6.4-dev (will bump to v0.6.5 after review)
* Migrate PGP wrapper for breaking OpenPGP.js api changes
* Change PGP wrapper decrypt api to return one signature arg
* Change emailDao and keychainDao calls respectively
* Fix tests
* Add tests keygen with empty passphrase
* Add tests to check PGP verification with wrong public key