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 find the name of the matcher too unspecific for what it does. I would the normal way as it is a lot more explicit:

`something.toString() should be "Something"`