Previously the code supplied `null` to BouncyCastle's
encryptor/decryptor builder's build method and that caused
NullPointerException to be thrown.
The fix checks if the passphrase is empty and omits the BouncyCastle
builder in that case.
Fixes#16.
This change removes two mechanisms that are deprecated in RFC 4880. The
spec explicitly mentions that "RSA Encrypt-Only (2) and RSA Sign-Only
are deprecated and SHOULD NOT be generated" [0].
The remaining RSA_GENERAL key type was renamed to just RSA for ease of
use for developers.
[0]: https://tools.ietf.org/html/rfc4880#section-9.1