Implement pair_blacklist functionality (#257)

* 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

freqtrade/misc.py

@@ -268,6 +268,14 @@ CONF_SCHEMA = {
                         'pattern': '^[0-9A-Z]+_[0-9A-Z]+$'
                     },
                     'uniqueItems': True
+                },
+                'pair_blacklist': {
+                    'type': 'array',
+                    'items': {
+                        'type': 'string',
+                        'pattern': '^[0-9A-Z]+_[0-9A-Z]+$'
+                    },
+                    'uniqueItems': True
                 }
             },
             'required': ['name', 'key', 'secret', 'pair_whitelist']