Provide a way to judge the current node instance is iojs

[lygstate]

We may do it by
platform.iojs === true

[lygstate]

node-gyp may beneficial from this, and for those apps that want to support for both io.js and node 0.12 would beneficial from this.

[kenany]

@lygstate So #493?

[lygstate]

Yeap, That's what I want.