Refactor to use subprocess not sh
Refactor to use subprocess not sh

Small refactor to use Python's 'subprocess' library rather than the 'sh'
library for external processes.  Also handle cower's very strange return
code behaviour.

vvoland/py3status,valdur55/py3status,ultrabug/py3status,valdur55/py3status,Shir0kamii/py3status,alexoneill/py3status,guiniol/py3status,Andrwe/py3status,Andrwe/py3status,ultrabug/py3status,tobes/py3status,docwalter/py3status,ultrabug/py3status,valdur55/py3status,guiniol/py3status,tobes/py3status,Spirotot/py3status