Mixpanel/register

`mixpanel.register({ $created: new Date })` doesn't work for some reason, mixpanel sends it as `{}` 0_o

1. `mixpanel.register({ $created: new Date })`

![mixpanel - meh](https://cloud.githubusercontent.com/assets/1661587/3908249/eda7eb42-2301-11e4-941a-14c9be0d0931.png)

2. `mixpanel.register({ $created: new Date().toISOString() });`

![screen shot 2014-08-13 at 6 49 35 pm](https://cloud.githubusercontent.com/assets/1661587/3908261/025fcd5c-2302-11e4-904d-7930fffd948f.png)

3. `analytics.identify(id, { created: new Date })`.

![analytics](https://cloud.githubusercontent.com/assets/1661587/3908267/11b6fbc2-2302-11e4-9b13-e1ed21a4d874.png)

cc @sperand-io @ianstormtaylor @DirtyAnalytics