Create an apkdownload-helper class

[mfonville]

a new "to be downloaded"-url would be added to the apkdownload-helper instance;
the helper would only do 1 download at a time per host
e.g.:
so 1 at aptoide can be parallel with 1 at apk-dl
but not 2 at aptoide at the same time

It could work with passing the avi-object (that contains the URL that needs to be downloaded) and we could make optional fields like "Stream=False" and "Header=''" to pass the necessary tricks.