Allow tracking of IP address

Hi,

I was trying to track events with an `ip` attribute like [so](https://github.com/adorableio/adorable-avatars/blob/master/lib/webserver.coffee#L56-L65), but mixPanel was ignoring that attribute.

By setting the request's `ip` property to `1`, I was able to get mixPanel to use the event's IP for geoloc purposes. This is hinted at [here](https://mixpanel.com/help/reference/http), under the "Special properties in events" section.

I didn't see an option for setting this in your library, so I added one. This was a quick and dirty implementation; please let me know if you'd like me to change it. It was also unclear how/if something like this would be tested, so advice in that regard would be appreciated as well.

Thanks!