Retarget project to specifically focus on App Tracking

Use Google's Measurement Protocol to report analytics, as this is required for
GA App properties. Some of the protocol has been reverse-engineered from the iOS
Google Analytics SDK, and much of the API has been refactored to match the API
provided in the iOS SDK. Future commits will bring the library up to feature-
parity with the iOS SDK including all tracking types and message queuing.

Use the Async Targetting Pack instead of System.Threading.Tasks (Mono) NuGet
package. Add new app-specific properties and access their values from the
platform-specific projects.
Reorganize projects, improve UserAgent, fix WinRT build, update readme