Enables the StyleCop rules for documentation in the NFluent project

Disables them in the NFluent.Tests project
Indicates the reasons of that choice within the Definition of Done manifest and also indicates why it is more likely to follow -when possible- the Java Fluent Assert methods names before proposing a new one within the NFluent library.
Adds a new HasSize() extension method with tests

Renames most of the unit test dealing with ExpectedException so that it increases names consistency
Fixes all the StyleCop warnings related to the documentation.