This also updates the version number to 3.0.0-beta4

Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform

This also updates the version number to 3.0.0-beta4

JSON tests fail, as we're not using OriginalNameAttribute yet.

- Spot an Any without a type URL
- In the conformance test runner, catch exceptions due to generally-invalid JSON

The conformance tests now use types which are part of src/google/protobuf, so we need to include src in the proto path.
The notes around "fix-ups" have been out of date for some time now.

Generated code changes for previous commit (basically InternalBuildGeneratedFileFrom => FromGeneratedCode)

Other changes are due to the well-known types changing without us regenerating.

Completely untested so far - easier to get started in VS and then transfer to Linux for tweaking...