use own version of CalculatePadding as it does no longer exist in Razor 3.0

When running with ASP.NET MVC default Razor engine, which is version 3.0 an error is thrown as the CalculatePadding no longer exists (neither does the base class that Span had that declared it).

I've added the method's implementation to the class with new name so it should now work with both Razor 2.0 and 3.0. Tested with Razor 3.0 and works now nicely with our templates.

Would appreciate an updated NuGet package.

Thanks for the great work you are doing with the project!