Fix behavior for North America, Russia & Kazakhstan (fix #7)

* Update list of North American area codes
  * Add Sint Maarten
  * Correct names for Bahamas, Dominica & Northern Mariana Islands
  * Order alphabetically
* Alter behavior of ItuCodes.find_by_itu_code so it only accepts valid country codes and not partial or full numbers
* Add a new method ItuCodes.country_for to look up a country by partial or full phone number
* Detect Russian and Kazakh phone numbers
* Remove private ItuCodes.lookup method
* Reorganize README
  * add more info on complicating factors
  * list main public methods
  * clarify expected arguments and returned values
Bump version to 0.4.5