[load]all: add Misc in load to get miscellaneous host info from /proc…

Add test for process.CPUTimes()

Document + add tests for for mem.VirtualMemory()

This change changes and documents the (previously undocumented) behavior of Used
to "RAM used by programs".

We also remove the undocumented and unused Shared field of that struct.

So with this change in place, the VirtualMemoryStruct contains:
* three human-consumable fields for Total, Used and Available memory
* one human-consumable UsedPercentage field
* a number of kernel specific fields

Use OS calls rather than exec() to get memory stats on Darwin

Before this change we used to exec() various binaries to find out memory
information.

While this worked, it was awfully slow.

And if somebody would want to compute how many percent of available memory all
PIDs on the system uses, that would take almost ten seconds on my laptop with
the previous implementation.

This implementation fares a lot better, and is smaller.

process: Implement MemoryPercent() for Darwin

It calls mem.VirtualMemory() and process.MemoryInfo(), so on the platforms where
those work MemoryPercent() will as well.

Tested on Darwin.

Add build check

add circle.yml.