[OHAI-412] Split Ohai::System responsibilites into 2 classes
[OHAI-412] fix uptime plugins for seconds_to_human change

Previously there was a _seconds_to_human method defined by the top level
uptime plugin, but this was moved to a mixin and the leading underscore
removed from the method name.
[OHAI-412] fix missing stubs in darwin kernel spec

Some popen commands weren't being stubbed so the tests fail on windows.
[OHAI-412] fix missing stub in linux kernel plugin

Was causing spec failures on windows
Update new specs for plugin class split

Rebased from 3967b5a3fd619dc392b8bf5dd2554d822a8fd400 to
d338698f288d08379bb36611f93a3a8db7ff45f5. Fix tests added in the interim
to correctly setup Ohai::System and plugin classes.