This commit adds support for VirtualMemory() in package mem. The support
only extends to total memory capcity, since that is all that is required
in Nomad. It does take into account global versus non-global zones, and
does not use cgo.
This has been tested inside a zone in the Joyent public cloud for a
non-global zone, and in the global zone of a SmartOS virtual machine.
This enables using gopsutil in a codebase that gets built on other OSes
than the ones supported. Instead of a build failure as before, due to
the build tags, gopsutil will now throw an "not implemented" runtime
error.
Fixes#234.
Justin Yang