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/blender/blender_engine.py

@@ -13,6 +13,16 @@ class Blender(BaseRenderEngine):
     supported_extensions = ['.blend']
     binary_names = {'linux': 'blender', 'windows': 'blender.exe', 'macos': 'Blender'}
 
+    @staticmethod
+    def downloader():
+        from src.engines.blender.blender_downloader import BlenderDownloader
+        return BlenderDownloader
+
+    @staticmethod
+    def worker_class():
+        from src.engines.blender.blender_worker import BlenderRenderWorker
+        return BlenderRenderWorker
+
     def version(self):
         version = None
         try: