Make nonce offset part of credentials file

[learner-long-life]

Currently, the nonce factory used can be different on each host from which a client calls the API, even with the same API key. This can allow one host/client to inflate the nonce, causing other host/clients to get errors back when calling the API.

The nonce offset hack to fix this is hardcoded in lib/poloniex.js but would be better off being part of the credentials file.

Future TODO: The credential file can also associate a hostname with each API key, to encourage better API key hygience (one API key per hostname).