[OS X] Add OS X support for most statistics

This pull request currently does OS X memory and load times, I cannot find a CPU usage function which works well with the usage percentage and load bar.

Suggestions are welcome and I will attempt to fix as I find the time.

I also attempted to add some *BSD support but it is more incomplete.

I figured I would make an incomplete pull request and expect that it would remain open rather than let it sit and other people would not get the benefit of this great tmux setup on OS X.