restructure coach and retry to fix nonce issues

[s4w3d0ff]

This is an attempt to fix #125 #122 and make the coach/nonces more thread friendly, more changes are needed, and this branch needs to be tested...

[s4w3d0ff]

If a nonce error occurs, the wrapper will automagicly update the nonce and try again. see #125

Python 2.7.12
[GCC 5.4.0 20160609] on linux2

>>> import poloniex
>>> import logging
>>> logging.basicConfig(level=logging.DEBUG)
>>> polo = poloniex.Poloniex('my-key', 'mysecret', timeout=None)
>>> polo._nonce = 0
>>> polo.returnBalances()['ETH']
INFO:requests.packages.urllib3.connectionpool:Starting new HTTPS connection (1): poloniex.com
DEBUG:requests.packages.urllib3.connectionpool:"POST /tradingApi HTTP/1.1" 422 None
DEBUG:poloniex:{"error":"Nonce must be greater than 1496260118086481. You provided 42."}
DEBUG:poloniex:PoloniexErrorNonce must be greater than 1496260118086481. You provided 42.
INFO:poloniex:-- delaying for 0s
INFO:requests.packages.urllib3.connectionpool:Starting new HTTPS connection (1): poloniex.com
DEBUG:requests.packages.urllib3.connectionpool:"POST /tradingApi HTTP/1.1" 200 None
DEBUG:poloniex:{"1CR":"0.00000000",,,}
u'0.00000000'
>>>

It should also handle/retry connection errors returned from poloniex (#122)