Fixed an incorrect CHECK in MesosSchedulerDriver::stop that assumes
'process' is always instantiated.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16592
Added freeMemory() to stout.

freeMemory() returns amount of free system memory. Can in
combination with memory() be used to get total used memory.
This allows future finer grained break-downs of used memory
in user memory, kernel buffers/caches and so on.

Review: https://reviews.apache.org/r/16569