CartManager::addItemByKey

- removed CartManager::addItem(key, value) and replaced with
CartManager::addItemByKey(key, value); PHP does not support operator
overloading
Merge remote-tracking branch 'origin/master' into cartfunctionality
Remove Item From Cart Closes #16

- users are able to remove any item from the cart regardless of the
items location in the cart
Merge pull request #25 from uptownjimmy/cartfunctionality

Cartfunctionality