Add xml-doc documentation for public API

[NameOfTheDragon]

I know documentation is a dirty word in the open source community, but the Should* extension methods could really benefit from some IntelliSense support in Visual Studio. I know I'm always at a loss whether to use ShouldEqual, ShouldBeLike or ShouldBeTheSameAs because the differences are not immediately obvious from the names. Breaking out of the zone to read the source code of a support library isn't really an optimal way to get documentation.

I would be happy to have a go at adding some XML doc comments if that is any help.

[robertcoltheart]

I'm working on bringing this project to life again, will look into adding some xml docs.