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.

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

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

We'll see what I've missed when CI fails...

ProtoDump isn't currently useful, but will be when ToString emits JSON: fixed.
ProtoBench: deleted; we should reinstate when there's a common proto3 benchmark.
ProtoMunge: delete; not useful enough to merit fixing up.

Removed the [TestFixture] from ByteStringTest as Travis uses a recent enough version of NUnit.

The solution as a whole doesn't build yet - we probably want to remove
ProtoDump and ProtoMunge entirely, and ProtoBench should use Jan's new
benchmarks for parity with Java.

The version of NUnit on my machine, packaged with Mono 3.12.1, is
only NUnit 2.4.2, which is extremely old - it still requires an explicit
[TestFixture] attribute on test fixtures. I've added one just for ByteStringTest
for the moment so that we can see some tests passing in Travis, but as part of
a separate PR we should work on making sure we're using a recent NUnit version.
(It may already be doing so, but we can check that once it's working and merged.)

Convert both the full and lite runtimes (and json/xml serialization assemblies) to be Portable Class Libraries.
All referring projects are now .NET 4 client rather than .NET 3.5.
This commit also fixes up the ProtoBench app, which I'd neglected in previous commits. (Disentangling the two sets of changes would be time-consuming.)

Move to a single solution file containing all of the C# projects, but no other solution folders - it's easier to edit those files outside VS than keep adding and removing them from the project.
The AddressBook protos have been regenerated (with a change to the example proto which I haven't included in this change - I'll wait for us to decide exactly what we're doing with namespaces before changing protos outside the csharp directory.

Note that now we've got Addressbook.cs which contains AddressBook and Addressbook classes. It's bad enough that we've got a class called AddressBook within a namespace of AddressBook (hard to get away from) but having things vary just by case is nasty.
This is more evidence that an option for renaming the file and descriptor class would be welcome. (A single option can probably handle both.)