Add portable .NET 4.5 library (profile 259)

Profile 259 supports .NET 4.5 + Windows 8 + Win Phone 8.1 + Win Phone + Silverlight 8.
This is a very wide and popular profile, also supported by Xamarin for Android and iOS. Google Analytics already has SDKs for those platforms, so I'm not sure how relevant they are for this project.

Added a console app to demonstrate how it can be used without any application frameworks like WPF, winforms and Windows store.

I'm not sure if I've approached this the right way, adding a ManualMeasurement class with platform-agnostic implementations for environment and system info and injecting an optional serializer by interface. It would be good to get some feedback on it.