Add googletest as a submodule in third_party/googletest.

We need to use a new enough version to ensure that constexpr is
available.

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.

"ps: StartFileDownload" is broken by a recent appveyor image update:
http://help.appveyor.com/discussions/problems/5308-ci-frequently-hangs-with-recent-image-update

Change-Id: Ia73aa8b12f77eb9bbec556a4c05310153aad97bf

"ps: StartFileDownload" is broken by a recent appveyor image update:
http://help.appveyor.com/discussions/problems/5308-ci-frequently-hangs-with-recent-image-update

Change-Id: Ia73aa8b12f77eb9bbec556a4c05310153aad97bf

Change-Id: I3d44e3c6877e00cfea5a903568fde1e6f597aac1

Change-Id: I3d44e3c6877e00cfea5a903568fde1e6f597aac1

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

Change-Id: Ieb7a2c2fbf35bc2a8fa65b915a5ecb68c83863e4