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
Only the sending is in the synchronized block, but the receiving is out of the block, can this ensure the requests reach the server side orderly? For instance, if we have 2 requests, any possible that the second request reaches the server side before the first request?