SystemInformation: Add GetLoadAverage() method

Implement on platforms with getloadavg() available.  Also implement
on Windows using an approach like that described here:

 http://stackoverflow.com/questions/23143693/retrieving-cpu-load-percent-total-in-windows-with-c

Co-Author: Betsy McPhail <betsy.mcphail@kitware.com>
Co-Author: Brad King <brad.king@kitware.com>
Change-Id: I5c50b964a05ece97e88f246df532ed6881bf2dce