Cyclic module dependency fix

Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1486, in __call__
    return self.func(*args)
  File "/home/artbot345/Downloads/Y13Computing-master/RegisterScreen.py", line 53, in <lambda>
    BackButton['command'] = lambda: self.goBack()
  File "/home/artbot345/Downloads/Y13Computing-master/RegisterScreen.py", line 83, in goBack
    app = LoginScreen(root)


Has been fixed.
See http://stackoverflow.com/questions/744373/circular-or-cyclic-imports-in-python and http://stackoverflow.com/questions/6351805/cyclic-module-dependencies-and-relative-imports-in-python to understand why this is needed.