add CipherState Rekey

[David Stainton]

as per latest noise spec section 5.1 the CipherState should have a Rekey:
http://noiseprotocol.org/noise.html#the-cipherstate-object

missing from here:
https://github.com/flynn/noise/blob/master/state.go#L16-L62

sad panda, want forward secrecy right now.
why diverge from latest noise spec?

[Jonathan Rudenberg]

Implemented in #15.