* Add ffmpeg_downloader.py
* Move shared download logic to downloader_core.py
* Added Windows support and some misc cleanup
* Fix issue with copying contents of .dmg files
* Add FFMPEGDownloader to engine_manager.py
* Add blender_downloader.py
* Add engine_manager.py
* Add additional methods to engine_manager.py
* Refactor file layout to make engines on par with workers
* Add system platform info to status response
* Default to using system platform / cpu if none are provided
* Add API to download an engine and some general cleanup
* Add method to delete downloaded engine
* Add API calls to download engines and delete downloads
* Misc fixes
