Differences in getIP() syntax

@tve sorry - it wasn't aimed at you... you didn't start the wiki and do the initial implementation. That was the slightly frustrating part :) Since then you've sanitised it quite a lot.

It's great that it takes an optional callback - that makes things a lot easier. It's a shame about the different argument types - but to be honest I should really update the other libraries to be more like the ESP8266 one in that respect.

With the first err argument - it's dumb, but it seemed to be what node.js did in most cases so I was trying to keep with that to keep things consistent, even if on some platforms err would never be used.