ENH: MeasureStartupTimes: Add option to flush disk cache

This commit adds the option "--drop-cache" to the script.

See http://unix.stackexchange.com/questions/87908/how-do-you-empty-the-buffers-and-cache-on-a-linux-system

Only supported only on Linux. If the --drop-cache option is used on
other platform, an exception is raised.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25111 3bd1e089-480b-0410-8dfb-8563597acbee