Add support for OSX

[genia]

Package works on OSX when added from github, but fails to install from npm.
Can you please add Darwin as a valid OS?

$ npm i snmp-native
npm ERR! code EBADPLATFORM
npm ERR! notsup Unsupported platform for [email protected]: wanted {"os":"linux","arch":"any"} (current: {"os":"darwin","arch":"x64"})
npm ERR! notsup Valid OS: linux
npm ERR! notsup Valid Arch: any
npm ERR! notsup Actual OS: darwin
npm ERR! notsup Actual Arch: x64

[bangert]

A PR for this would receive my support. Thanks