Add class for handling downloads
Add class for handling downloads

====  Tech Notes ====
Downloader() is a Thread subclass starting and handling a download. The
property done and status is set when the download is completed or
failed.
download manager ensuring continuation of started downloads (per process)
complete_action, a function to be called after download is complete can be
supplied to the downloader

Dark5ide/mycroft-core,forslund/mycroft-core,MycroftAI/mycroft-core,linuxipho/mycroft-core,aatchison/mycroft-core,Dark5ide/mycroft-core,MycroftAI/mycroft-core,aatchison/mycroft-core,forslund/mycroft-core,linuxipho/mycroft-core