Add support for iOS contact groups

Contacts on iOS are organized by group. There is the default group "All Contacts" plus any custom groups that the user set up. Contacts retrieved by Apache Cordova / phonegap do not contain which group they are in.

CONTACT CATEGORIES PROPERTY

I believe that iOS groups are analogous to phonegap contact categories and thus should be mapped to them. If not, another property should be created.

From the Cordova docs:

"categories: An array of all the contacts user defined categories. (ContactField[])"

IOS IMPLEMENTATION NOTES:

In the iOS AddressBook API, the function ABAddressBookCopyArrayOfAllGroups() gets the list of groups.

https://developer.apple.com/library/ios/#documentation/ContactData/Conceptual/AddressBookProgrammingGuideforiPhone/Chapters/DirectInteraction.html#//apple_ref/doc/uid/TP40007744-CH6-SW1