Adds page and screen to Amplitude

This pull request adds the `page` and `screen` methods to the Amplitude integration with naming parity with iOS and Android. 

It also upgrades integrations to the most recent spec version of facade, which has a global `.traits()` object to make page and screen calls useful in Amplitude.
we can use [`page.event`](https://github.com/segmentio/facade/blob/master/lib/page.js#L85-L89) and [`screen.event`](https://github.com/segmentio/facade/blob/master/lib/screen.js#L52-L56)