Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

 Add unknown field support for csharp

Note that the compatibility tests have had to cahnge as well, to
cope with internal changes. (The test project has access to
internals in the main project.)

Fixes #3209.

This has one important packaging change: the netstandard version now
depends (implicitly) on netstandard1.6.1 rather than on individual
packages. This is the preferred style of dependency, and shouldn't
affect any users - see http://stackoverflow.com/questions/42946951
for details.

The tests are still NUnit, but NUnit doesn't support "dotnet test"
yet; the test project is now an executable using NUnitLite. (When
NUnit supports dotnet test, we can adapt to it.)

Note that the project will now only work in Visual Studio 2017 (and
Visual Studio Code, and from the command line with the .NET Core
1.0.0 SDK); Visual Studio 2015 does *not* support this project file
format.

All the files are copied from 3.0.0 (JosnFormaterTest was deleted)