[Improvement] Added spam limit protection.
[Improvement] Added a check so you cannot call "send" multiple times.
[Improvement] setRecipient now only takes RecipientInterface instance
[Improvement] Recipients are replaced instead of appended by default.
[Improvement] Added a check for when adding duplicate recipients to the same notification.
[Improvement] Email class now implements RecipientInterface.
[Improvement] Added RecipientInterface which all recipient classes need to implement.