Log stderr

With this PR, i3pystatus wil lcreate a logfile .i3pystatus-<PID> when a module has something to log. If there is nothing to log, then no file is created (thanks to delay=True parameter). I've updated the mail and shell modules to log stderr to this file (if the "enable_log" setting of the module is true). 
It solves https://github.com/enkore/i3pystatus/issues/110