Handle long output from arp

[rishabhtulsian]

If the output from arp is long for example (256 lines), the on 'data' for stdout is called multiple times and the complete arp table is not returned.

I have a fix for it this pull request #5

[skepticfx]

Thanks @rishabhtulsian

I've merged it now. If you're interested I can add you as a moderator as well, so you can commit directly :) Let me know.