Implemented the base structure of an MVC
- implemented the functionality that allows to perform queries by defining new classes that extends the Model class
- implemented the login functionalities
- changed the ddl.sql that defines the structure of the database
- defined some models like user, note and course, that are associated to 'users.users', 'courses.notes' and 'courses.courses' tables
- redifined the 404.html and 500.html pages in order to be included inside the application layout, that allows to have a single layout
  for all the pages
- updated bootstrap to the third version
- created the users, notes and courses controllers