Add more functions in the Should API, and add the Must API.

In one of my projects, I have coded several functions that are relevant to the Should class.
We should discuss the implementation details on several of them, such as `should_beJnarioLink`.

I also added the Must class. It is based on Should but makes assertions on the results given by Should. This class is helpful for writing specification facts.
I am curious, what are you using these matchers for? They seem to be pretty specific and I would like to avoid having these in the normal Should class. I would prefer putting these into a separate class.