* remove obsolete helper functions and make _state a public member.
* remove function assertions
* revert worker() changes
* Update pytest from 3.4.2 to 3.5.0
* Adapt exchange functions to ccxt API
Remove get_market_summaries and get_wallet_health, add exception handling
* Add NetworkException
* Change pair format in constants.py
* Add tests for exchange functions that comply with ccxt
* Remove bittrex tests
* Remove Bittrex and Interface classes
* Add retrier decorator
* Remove cache from get_ticker
* Remove unused and duplicate imports
* Add keyword arguments for get_fee
* Implement 'get_pair_detail_url'
* Change get_ticker_history format to ccxt format
* Fix exchange urls dict, don't need to initialize exchanges
* Add "Using Exchange ..." logging line
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