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
`Returns tuple boolean (success)/ string (error msg, output)` but you return `out, result`.

I don't get line 27/28: why chop a single space of the end?