ValueFactory (nonce)

These commits introduce ValueFactory interface and a simple implementation for it, based on System.currentTimeMillis().

The purpose is to calculate nonce just before sending it, both in a single synchronized block.

There are simple unit tests, I have also tested it on a live exchange. https://github.com/rafalkrupinski/XChange/tree/nonce-kraken

also see https://github.com/timmolter/XChange/issues/525
Use the ValueFactory instance as the synchronized lock?