move cache folder from ROOT/_cache to tempdir determined by tempfile module

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
create a subfolder in the temp directory for coursera-dl

use the style inspired by: https://github.com/jplehmann/coursera/pull/183/files

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>
Merge pull request #188 from arlimus/master

Change the location of the cache directory to not be put on the location of
the executable pieces, as that prevents users from installing coursera-dl in
a system-wide fashion.

Instead, use the appropriate system facilities to create temporary files, as
per the `tempfile` module.

This is the result of a cross-polination of fertile discussions among
@jonasdt, @arlimus, and @euyuil.

This closes #183 and #188.


Signed-off-by: Rogério Theodoro de Brito <rbrito@ime.usp.br>