Add detection for IPv6 addresses in Trace#fromRequest

[cjthompson]

This patch is based on the other pull request to not crash on an IPv6 address. With this patch, node-tryfer is now compatible with node 4.x and 5.x. I fixed the linting issues from the previous patch and leverage the node built-in net.isIPv4 function with a fallback to a custom implementation.

One additional tweak I added was to check for IPv6 mapped IPv4 addresses and extract the IPv4 address:

    // Check IPv4 mapped IPv6 addresses
    if (/^::ffff:(\d{1,3}\.){3,3}\d{1,3}$/.test(host.address)) {
      host.address = host.address.replace(/^::ffff:/, '');
    }

[9point6]

Is there any recent progress on getting this merged?

[rooftopsparrow]

Bump.

[Kami]

I quickly looked over the changes and they look good to me.

Proper review would be better though, but I don't know when I will be able to get to it. Having said that, we are happy to give write access to new people who would like to help and maintain the project.

[Kami]

Some unit tests for this function would also be good.

[Kami]

Sorry for the delay.

I did a more in-depth review and the changes looked good to me so I went ahead and merged them into master.

Sorry for the delay again and thanks a lot for this great PR.