Support for multiple versions of the same assembly

[MikaBerglund]

We need to support multiple versions of the same assembly and produce documentation for them in the same application. This is highly useful in cases where you have a published production version of an assembly, but include documentation also for a preview version of the next version of that same library.

This has to be done so that the DocumentationParser would take the XML documentation and assembly data as constructor parameters, and work only with that data, and not require that the assembly is loaded into the app domain of the current application.

This might require quite a bit of redesign, but at this point it is much easier to do, since we are still working towards the first version. And this will probably be quite an important feature.