makes basic memory display work on OSx 10.8.x by using psutil's provided available attr

The `cached` attr used to work on BSD/OSx but no longer does. However psutil provides a
useful `available` attr that seems to do the same calculation taking into account platform
differences.  According to psutils docs `available` is "the actual amount of available
memory that can be given instantly to processes that request more memory in bytes; this is
calculated by summing different memory values depending on the platform (e.g. free + buffers
    + cached on Linux) and it is supposed to be used to monitor actual memory usage in
a cross platform fashion.".

via http://code.google.com/p/psutil/wiki/Documentation#Memory