Improved interpretation pipeline & subroutine definitions.
Added conditional statements (if, else, endif core words).
Added associated test methods within unit test.
Few minor changes.
Added additional if conditional test methods.
Removed erroneous precondition in D4Var.Scope.newVar().
Added true and false boolean constants (core words).
Added associated test method.
Added D4Var.toString, D4Var.Stack.toString() and D4Var.Hash.toString().
Added associated test method in unit test.
Changed to better use D4Var.toString().
Finalised unary operator core word handler and added unit test.
Added associated test method in unit test.