Google Analytics: add support for sending data from server to "app" properties 

*Background*: Our implementation of the GA measurement protocol dictated that we could only send data to "web" properties. This meant that our Xamarin users, anyone who wanted to use our "light" mobile SDKs, or anyone proxying their app data through their own servers and using our server-side libraries (lots of cordova users) had a degraded experience. They had to use GA web properties to look at their app data. Now they can use app properties!

Turns out the reason this was the case was because we were only mapping page calls and sending "pageviews," and app views expect "screenviews."

https://developers.google.com/analytics/devguides/collection/protocol/v1/devguide#screenView

Quick video with some end to end testing (1 screenview 1 event) showing this working as expected: https://cloudup.com/cx-DG5xIl5B

thoughts? happy to field any questions. @amillet89 @calvinfo @ianstormtaylor @DirtyAnalytics @yields