core/cachew: rely on ~/.cache for default cache path

- rely on appdirs for default cache path instead of hardcoded /var/tmp/cachew
  technically backwards incompatible, but no action needed
  you might want to clean /var/tmp/cachew after updating

- use default cache path (e.g. ~/.cache) by default
  see https://github.com/ActiveState/appdirs#some-example-output for more info
  *warning*: things will be cached by default now (used to be uncached before)

- treat cache_dir = None in the config
  *warning*: kind of backwards incompatible.. but again nothing disasterous

tox.ini

@@ -22,6 +22,7 @@ commands =
 setenv = MY_CONFIG = nonexistent
 commands =
     pip install -e .[testing]
+    pip install cachew
 
     hpi module install my.location.google
     pip install ijson # optional dependency