Converted AssemblyVersion attribute to a central GlobalAssemblyInfo.cs file.

This simplifies future version number updates, although the nuspec files still require their own version edits. It also would allow other common attributes (like AssemblyCopyright) to be moved to this file in the future, and no longer duplicated across all projects' AssemblyInfo.cs files.

I did not touch the projects under tests, not wanting to reference ../../src/GlobalAssemblyInfo.cs from them, although that could be done, or this file could be moved up another level, or a second GlobalAssemblyInfo.cs file could be created under tests.