Run tests on .NET Core runtime and framework

This change uses the Microsoft.DotNet.TestHost package to run test
assemblies against a test version of .NET Core, rather than against
the locally installed version of the .NET Framework.  It also adds
the build property "SkipTestAssemblies".  SkipTestAssemblies can be
set to a semicolon delimited list of test assemblies to suppress.
For example:
build.cmd /p:SkipTestAssemblies="System.Linq.Parallel.Tests;System.Col
lections.Immutable.Tests"
Below the nuspec has a dependency on 1.0.1-prerelease and here you depend on 1.0.0-prerelease? Why do we hard code the version? Shouldn't this depend on what we dependency we include the NuSpec?