* This allows us to build with the .NET Core tools, currently targeting
  .NET Standard 1.6
* We use a separate build tree under cs/dnc so that we don't interfer
  with the MSBuild-based build. This tree refers to the same .cs files
  as the MSBuild-based build.
  * Sometime soon, .NET Core tooling will switch to using MSBuild. At
    that point, we'll be able to integrate the two builds.
* Much of the generated code is simply checked in right now. A later
  commit will switch to actually running code generation.

* This adds .NET Standard 1.0 support for all assemblies exception
  Bond.IO.
* The lowest standard that Bond.IO can target is .NET Standard 1.3, so
  we target that.

* Removed checked in generated code.
* Uses the same MSBuild codegen targets that we ship in the NuGet
  packages.
* Add a little build.ps1 script to automate the build.

* Build script can be used to pick the configuration.
* Delay signing--like we use for official Bond releases--can be enabled
  by providing a delay sign key. The official Bond delay sign key is NOT
  part of this repository.

* Enable script strict mode.
* Be more robust in the face of errors by setting $ErrorActionPreference
  to 'Stop' so that errors cause an immediate failure.
* Enable the build script to be called from a directory other than the
  cs/dnc directory.

This collects up all the assemblies needed in one directory tree,
similar to the copying that is done for targets with
$BondRedistributable==true by the MSBuild's Common.Internal.targets

Instead of using .NET 4.5 builds for .NET Standard 1.0, we now use the
results of the .NET Core builds for the various versions of the standard
that are supported.

* Added to AppVeyor matrix
* Added -Verbosity and -MSBuildLogger switches to .NET Core build script, as
  we want to specify both of these for AppVeyor

* Update required tools with what's needed to build the .NET Core
  version.
* Document which assemblies are available for which frameworks.
* Add cmd wrapper around the .NET Core build script so that it works
  whether or not PowerShell is being used directly.
* Update CHANGELOG