Improvements on the tox upload/download script

[tarekziade]

Remy suggested to use localhost instead of kinto.dev in https://github.com/Kinto/kinto-attachment/blob/master/tox.ini#L26

[tarekziade]

Another improvement would be a better handling of the temp space. e.g. create a python script that deals with creating a temp directory where we put everything, then clean it up. using /tmp is not robust