Add the concept of an ElePHPantCollection

[ramondelafuente]

The Herd aggregate is in charge of keeping ElePHPants. But when it contains ElePHPants, it keeps them in an array internally, meaning there is some logic in the Aggregate methods regarding adding/removing an ElePHPants of a certain Breed.

That logic can be contained in an ElePHPantCollection, which is the only thing the Herd needs to have to do it's job.

• Create the (phpspec) specifications for an ElePHPantCollection, and write the implementation.
• Switch the internal array to the new ElePHPantCollection inside the Herd aggregate