default db-engine to master if context is missing

Currently if the db is invoked without app_context flask_replicated
fails with a RunTimeError. Instead default to the master if app_context
is missing.

Author: David McLean

flask_replicated.py

@@ -22,7 +22,7 @@ def init_app(self, app):
                 get_engine_vanilla = db.get_engine
 
                 def get_replicated_engine(app=app, bind=None):
-                    if bind is None:
+                    if bind is None and g:
                         use_slave = getattr(g, 'use_slave', False)
                         use_master = getattr(g, 'use_master', False)
                         if use_slave and not use_master:
@@ -42,6 +42,7 @@ def changes_database(func):
     func.use_master_database = True
     return func
 
+
 def use_slave_database(func):
     func.use_slave_database = True
     return func