* Adding an optional black_list of pairs not to be traded
* applying the blacklist also when not using --dynamic-whitelist
* fix error retrieving pair in conf
* Refactoring the handling of whitelist among the various functions
* unit test to verify that black listed pairs are being removed from the pair_whitelist
* Fixing newly added unit tests in develop
* fixing flake8 code review
* fix code review from @garcq