Added partial form to avoid code repetitions in the new todo and edit todo views

new file:   app/views/todos/_form.html.erb
modified:   app/views/todos/edit.html.erb
modified:   app/views/todos/new.html.erb
Optimized index view to avoid the conditional statement
modified:   app/views/todos/index.html.erb

Added a method in the model to group the list of todos by list name
Coorected the controller accordingly
modified:   app/controllers/todos_controller.rb
modified:   app/models/todo.rb
Refactored the model.
Merged all the status into a frozen hash.

created dynamicaly the associated methods
modified:   app/models/todo.rb