81 cardio and age fields

Closes #81 

To test, get a user from SiteUser and an exercise from AerobicExercise.  Verify that the calories_burned field of the exercise object is the same as the output of this formula http://fitnowtraining.com/2012/01/formula-for-calories-burned/

Then, make a new, female SiteUser and an AerobicExercise for that user (Which will involve making a workout for that user) also make an entry in Weight for the user.  Then verify the output of calories_burned based on the formula again.