Adds MacOS-specfic ps aux command.

[johnnymatthews]

Potentially closes #89

[casperdcl]

I think actually we can drop the -

Suggested change

	else
	if is_osx; then
	load=$(cached_eval ps aux | awk '{print $3}' | tail -n+2 | awk '{s+=$1} END {print s}')
	cpus=$(cpus_number)
	echo "$load $cpus" | awk -v format="$cpu_percentage_format" '{printf format, $1/$2}'
	else
	load=$(cached_eval ps -aux | awk '{print $3}' | tail -n+2 | awk '{s+=$1} END {print s}')
	else
	load=$(cached_eval ps aux | awk '{print $3}' | tail -n+2 | awk '{s+=$1} END {print s}')