This potentially avoids compiler warnings.

* Changing DOTNET35 framework symbols in preprocessor directives to the default built-in value of NET35.
* Adding extension method StreamExtension.CopyTo for .NET 3.5 because it didn’t exist until .NET 4, and adding associated unit tests.

This is the only call to TypeExtensions.IsValueType, so we can remove
that method, making the whole type conditionally compiled out for .NET 3.5

* Adding condition compiler symbol to support .NET 3.5

- Fix nupec paths
- Remove an obsolete part of the JSON build
- Add documentation and tests to reflection extension methods, and improve implementations

This requires .NET 4.5, and there are a few compatibility changes required around reflection.
Creating a PR from this to see how our CI systems handle it. Will want to add more documentation,
validation and probably tests before merging.

This is in aid of issue #590.