Add xunit assemblies to the repository.
Move common project configuration settings into shared files.

Common build configuration properties are moved into build\ScriptCs.Common.props, which is imported by each *.csproj file in the solution.

Common assembly attribute declarations can be found in CommonAssemblyInfo.cs and CommonVersionInfo.cs.
Add a build script to the solution.

#26
Merge pull request #75 from jrusbatch/build-script

Create a build script for the solution and consolidate project configuration into shared files.