[1/3] Add support for multiple exchanges with ccxt (objectified version) (#585)

* 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

requirements.txt

@@ -12,7 +12,7 @@ scipy==1.0.0
 jsonschema==2.6.0
 numpy==1.14.2
 TA-Lib==0.4.17
-pytest==3.4.2
+pytest==3.5.0
 pytest-mock==1.7.1
 pytest-cov==2.5.1
 hyperopt==0.1