Change SimpleJson to be a static class

The class has no instance members, thus it should be a static class. SimpleJson's README lists .NET 2.0 as the minimal supported version. Static classes are supported in C# 2.

Also, when I add the library to my project, I get a lot of FxCop violations. I'll send a PR with more suppressions and/or fixes.

Any plans to drop .NET 2.0 support ever?