Added API for Aerobic Exercises

Closes #74 

To run do the following:

sh reset_db.sh
./manage.py shell
(Within shell)
run populate.py
(Close shell)
./manage.py runserver

Go to http://localhost:8000/api/list/aerobicexercises/1/
Try adding an exercise

Go to http://localhost:8000/api/get/aerobicexercises/1/
Try updating and deleting the exercise