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/ffmpeg/ffmpeg_engine.py

@@ -7,6 +7,16 @@ class FFMPEG(BaseRenderEngine):
 
     binary_names = {'linux': 'ffmpeg', 'windows': 'ffmpeg.exe', 'macos': 'ffmpeg'}
 
+    @staticmethod
+    def downloader():
+        from src.engines.ffmpeg.ffmpeg_downloader import FFMPEGDownloader
+        return FFMPEGDownloader
+
+    @staticmethod
+    def worker_class():
+        from src.engines.ffmpeg.ffmpeg_worker import FFMPEGRenderWorker
+        return FFMPEGRenderWorker
+
     def version(self):
         version = None
         try: