Various Improvements

• Made the XCTestCase.testRequirements(in directory:String? parameter default to nil for the most common case of just discovering all requirements

• Call the beforeEachExample closure before each Example is run

• StatementHandler can now be created with one of more String statements to match

• Fixed the static factory methods on StatementHandler to take throwing closures

• Added factory method overloads for accepting handler closures that don’t use any input. This is a very common case that should always require explicitly ignoring an input parameter, e.g. { _ in }