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.
Should uses a custom Assert class in order to avoid having explicit dependencies to JUnit. This way Jnario is independent of the actual testing framework and it is easier to port it to other test frameworks such as TestNG.