Implement BitexAuth

[deepbrook]

The Auth class should default to doing nothing.
In addition, a nonce() method, which returns a millisecond ts as a str needs to implemented as a static method.

Additionally, properties to convert secret and api key to bytes on the fly is useful.

Finally, a method to unparse the passed request's body; this may be required by some exchanges, as they generate their signature from the request's parameters.