Add system wide network protocol stats

[nathanielc]

A new method NetProtoCounters has been added that returns a list of NetProtoCountersStat objects.
A map is used in NetProtoCountersStat since the stat names are different per protocol.

Typical contents of /proc/net/snmp
The file contains system wide counters about different network protocols.

Ip: Forwarding DefaultTTL InReceives InHdrErrors InAddrErrors ForwDatagrams InUnknownProtos InDiscards InDelivers OutRequests OutDiscards OutNoRoutes ReasmTimeout ReasmReqds ReasmOKs ReasmFails FragOKs FragFails FragCreates
Ip: 1 64 145831951 0 4 420174 0 0 145410079 138817441 3718096 285 0 0 0 0 0 0 0
Tcp: RtoAlgorithm RtoMin RtoMax MaxConn ActiveOpens PassiveOpens AttemptFails EstabResets CurrEstab InSegs OutSegs RetransSegs InErrs OutRsts InCsumErrors
Tcp: 1 200 120000 -1 441395 303349 29634 9431 66 14558102 12577486 14364 676 53622 0
Udp: InDatagrams NoPorts InErrors OutDatagrams RcvbufErrors SndbufErrors InCsumErrors IgnoredMulti
Udp: 67688650 3723361 58108572 124599763 58108572 0 0 318767

Only linux support has been added. Other OSes have been give an implementation that returns a not implemented error.

Fixes #108

[sparrc]

This would be great! 👍

[shirou]

Awesome!! Thank you very much for this beautiful contribution.

It looks so good to me but I want one thing.
Could you add something to README? (My English is poor to represent your work.)

[nathanielc]

@shirou Updated README... thanks for looking at this so quickly

[shirou]

Thank you very much!!