Fixed broken memory calculation for OSX Mavericks

In Mavericks the output of the top -l 1 command used changed from

PhysMem: 300M wired, 760M active, 497M inactive, 1557M used, 490M free.

to

PhysMem: 5034M used (1335M wired), 2402M unused.

This broke the memory calculation. Another elsif was added to the
darwin_top function to address this issue.