Use tmp dir or equivalent for downloads

[myselfhimself]

On linux, with blender-addon-tester 0.1 installed in a virtualenv, when trying to run BAT, if in my home directory, since the default download path is ../, I get a permission denied related to writing.
Some sort of temp directory should be decided cross-OSes. Python does have mktemp, but such a temp directory must be redetected / reused somehow every time the script starts again... which might mean using instead home settings.