Added the ability to fetch an alternative location for /proc via an
environment variable. If the env var is not set it will return /proc as
the default value.

shirou/gopsutil#90 make hostinfo more robust

cpu[darwin]: separetes cpu_darwin into cgo and nocgo to get CPUTimes.

CreateTime() on linux returning an incorrect timestamp

Fix docker on Centos 7

When MemAvailable is in /proc/meminfo, use it (kernel 3.14+)

Use vm_stat command on darwin to get memory

This has the benefit of allowing us to also grab the cached memory so
that we can get an "available" measurement

process[linux, darwin]: implements Connections using lsof.

Support 32-bit linux, this field is int32 on 32-bit systems

Add process/Parent, net/NetConnections on darwin and linux

Add common.invoker interface to mock exec.Command. common.FakeInvoker returns
expected file if exists instead of invoke exec.Command.
Currenly, mocking is enabled only process.Pids(). I will expand to other funcs incrementally.

Detect error in CPUTimes fails on callPs, and modify comments on ReadLines

Ensure that process.MemoryInfo returns a up-to-date value.