Change Guice version to 3.0.
Add one composite measurement to another.
Add ShoppingListItem class.
Add ShoppingList class.
Replace Unit enum with Strings.
Iterate over TextRepresentable objects in CompositeMeasurement#getText.
Rename add methods in CompositeMeasurement.
Rename add method in SingleMeasurement.
Remove duplication between ShoppingList and CompositeMeasurement add methods.