Engine and downloader refactoring (#50)

* Make downloaders subclass of base_downloader.py

* Link engines and downloaders together for all engines

* Replace / merge worker_factory.py with engine_manager.py

src/engines/core/base_engine.py

@@ -21,7 +21,7 @@ class BaseRenderEngine(object):
 
     @classmethod
     def name(cls):
-        return cls.__name__.lower()
+        return str(cls.__name__).lower()
 
     @classmethod
     def default_renderer_path(cls):
@@ -39,6 +39,14 @@ class BaseRenderEngine(object):
     def version(self):
         raise NotImplementedError("version not implemented")
 
+    @staticmethod
+    def downloader():  # override when subclassing if using a downloader class
+        return None
+
+    @staticmethod
+    def worker_class():  # override when subclassing to link worker class
+        raise NotImplementedError("Worker class not implemented")
+
     def get_help(self):
         path = self.renderer_path()
         if not path: