Adding "getContactsVcard" method to APAddressBook

I added a method that uses Apple's native method "ABPersonCreateVCardRepresentationWithPeople"

Using this method you can get the vCard string for a single or multiple contacts.

I also added an option to not get the "PHOTO" inside the vCard (this really makes the string very large and a lot of users will not want the photo in the vCard"

https://en.wikipedia.org/wiki/VCard

Let me know if you want me to add anything else before

PING

Thank you for pull request. I'll check the code a bit later.

Any updates?

Sorry, still not enough time.

Do you have an example of using this vCards? Actually I don't understand what it is)) Also, please read contributors guide