Debug statements

When quick and hacky println statements are what you want, the Debug trait, mixed in every Specification, provides useful methods:

• pp or “print and pass”, prints a value to the console, then returns it to be used in the rest of the expression: graph.pp must haveSize(3)
• pp(condition) prints a value if a condition holds
• pp(f: T => Boolean) prints a value if a condition on that value holds