add deterministic tests and test vectors

[david415]

"other language implementations of secret-handshake should share these test vectors. this would give implementers some confidence that they've done it correctly. additionally the binary compatibility among language implementations might prove useful."

https://github.com/auditdrivencrypto/secret-handshake/issues/5

[cryptix]

agreed! would synchronize this with auditdrivencrypto/secret-handshake#6