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.