Minor changes to the breed/breed_repo class.

In the breed class, added the id property.
Id property not assumed to be given in the object. Instead, the find method in breed_repo will find the id and return it.
Id added for the purpose of the Puppy class being able to reference the breed by id.
Added functionality: find method returns false if search result was nil for that breed.
Added tests: returns nil for breed not in database.
breed_repo and breed passed new and existing tests.