Fix perfdata human

Use `format_number()` on warning, critical, minimum and maximum. Doing it just on the actual value returned by the plugin does not look as good nor is it as helpful.

Before:

![before](https://cloud.githubusercontent.com/assets/3237256/4112179/42348582-321e-11e4-942e-98a2fca0e67e.PNG)

After:

![after](https://cloud.githubusercontent.com/assets/3237256/4112180/454989a2-321e-11e4-858e-93e381461de1.PNG)