Add modifiably Nonce

[Trevypants]

Describe the solution you'd like

Currently the code has a pre-defined nonce as follows:

params["nonce"] = str(int(time.time() * 100_000_000))

In most cases, this nonce definition poses no problems. However, in integrated applications, it could be that an individual would like to define their own custom nonce function. I believe that it would be very helpful for users to be able to provide their own custom nonce function.

This has the extra benefit in helping avoid invalid nonce errors with asynchronous API calls. When performing concurrent API calls, it could be that the nonces are just off (not sequential) that Kraken returns an invalid nonce error. However Kraken offers a nonce error window that allows for a certain leeway in the nonce values provided (which can work well with asynchronous API calls). However, the nonce error window is dependent on the nonce function. The custom nonce function can help users define these windows so asynchronous API calls can work.

A possible solution implementation is to include a nonce function input on the client initializer which is used on every API call.