Multi framework targeting

The changes in this PR unify the master and NET_40 branches into one.  Instead of having two branches, we will maintain one code base that can be compiled against the 4.0 and 4.5 .NET frameworks.  The build files follow the pattern used in other multiple framework projects like ZocUtil.  These will then be packaged into a NuGet package that Scout, our automated deploy tool, can reference.  We can then put that package on nuget.org.

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