Changed all models from being Singletons to standard classes

Controllers do not need to share any of the model data with each other,
so it is more appropriate for controllers to instantiate new model
objects if necessary. Therefore, Singletons have been changed to
standard classes that can be instantiated multiple times.

This now allows users to create more than one note.

Signed-off-by: Byron Sanchez <bfs5030@gmail.com>