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

Fixes #2088.

We now have separate tests for netcoreapp and net45 to test the two branches here.
(netstandard10 doesn't have MemoryStream.GetBuffer)

Although most of this library doesn't have any async functionality,
this feels like a natural place to locally add it.

Fixes #2511.

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

- We do still generate the message types, as otherwise reflection breaks, even though it doesn't actually use those types.
- JSON handling hasn't been implemented yet

Everything should now be consistent - the only difference between files is the year (aside from generated files).

Fixes issue #531.

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.)

This includes the NUnit test adapter which allows NUnit tests to be run under VS without any extra plugins.
Unfortunate the compatibility tests using the abstract test fixture class show up as "external" tests, and aren't well presented - but they do run.

This includes fetching the VS unit test runner package, so that tests can be run from Visual Studio's Test Explorer.