Convert to unified API

This is currently pointed at the `3.5.0-alpha2` NuGet packages for MvvmCross so we may want to hold off merging this in before that's more stable, but I wanted to at least get this open to track the changes. Not much really had to change, mainly the normal boilerplate changes when migrating to the unified API. I also removed the `packages` folder from the repository while I was at it to cut down on the noise :smile: