Remove dependencies on Portable Class Library

Create C# library projects for OxyPlot_NET40 and ExampleLibrary_NET40
Update OxyPlot.AppVeyor.sln to include the two new projects library
Update projects referencing OxplyPlot and ExampleLibrary to handle the NET_40 compatibility
Update the OxyPlot.Core.nuspec to handle .NET4.0 and .NET client profile frameworks version