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'm using Jnario for writing reference documentation. Some specifications refers to other reference documents (in other specification files).
I had encountered link validity problems in the generated HTML when I change the name of the specification files or the fact labels for examples.

This tool allows me to check that the URL typed in the Markdown is valid and referring an other element from the Jnario specification set. Indeed, I want to check all the things that I have written in my natural language explanations are true.

A concrete example could be found in: 
https://github.com/sarl/sarl/blob/master/docs/io.sarl.docs.suite/src/test/java/io/sarl/docs/reference/GeneralSyntax.spec (search for `Jnario` string).

Of course, if you have another way to verify the URL typed in the Markdown text, it is walcome :smile: