Bug Fix: towards and distance

both towards and distance should optionally take a turtle as a parameter rather than two numbers.
trailing whitespace removed
Add a NotImplementedException

To improve the error reporting, I created a NotImplementedException.  Now super() throws this exception rather than giving a rather unintelligible Name error about super_$rw$ not found.  This should be expanded to all of the builtin functions that are not yet implemented in skulpt.