ECDH.generatePublicKey can get the public key using the curve
and private key. This allows usage of the crypto module where
a developer imports a private key, and then generates a public
key without needing it stored.

Removed the generated_ boolean from the ECDH class, and stopped
checking for it with getPrivateKey() and getPublicKey(). This
allows you to import public and private keys without having to
generate first, which would just rewrite the generated keys
regardless.

An error message was changed to accurately reflect what the error
was.

Change the function name of generatePublicKey to be 
generatePublicKey instead of generateKeys.