add scraper for OKEx exchange

[PumpkingWok]

Description

• This PR add the scraper for OKEx exchange using websocket.
• It includes the functions to reconnect to ws and subscribe again to all channels in case of connection error. This happens frequently.
• There could be a code error into dia/config.go in GetConfig() func, when it istantiates configFileApi, it should forgot .json at the end of string creating, right ? Every exchange scraper catch the error.
• I used UTC timestamp, it has created during runtime immediately after retrieve the order. I have not used the timestamp into response because it has only the hour, It could be ambiguous. I tested it and the max difference is 2 seconds.

Checklist

• Changes don't break existing behavior

Affected core subsystem(s)

Scraper Core

Testing

I tested the code in MAC OS environment using go 1.10.3

Refers/Fixes

FIxes: #4

[frranck]

Very good, I will have a look how we can skip this error message for exchanges that don't need an api key.
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