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

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.)