[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

freqtrade/tests/testdata/download_backtest_data.py

@@ -6,7 +6,7 @@ import sys
 
 from freqtrade import exchange
 from freqtrade import misc
-from freqtrade.exchange import Bittrex
+from freqtrade.exchange import ccxt
 
 parser = misc.common_args_parser('download utility')
 parser.add_argument(
@@ -28,7 +28,7 @@ PAIRS = list(set(PAIRS))
 print('About to download pairs:', PAIRS)
 
 # Init Bittrex exchange
-exchange._API = Bittrex({'key': '', 'secret': ''})
+exchange._API = ccxt.bittrex({'key': '', 'secret': ''})
 
 for pair in PAIRS:
     for tick_interval in TICKER_INTERVALS: