add Kuwait w/test, and allow for blank area codes

### WHAT

* adds Kuwait to country list by giving it an `:area_code` and `:local_number_format`
* add blank `:mobile_number` to Kuwait so it doesn't default `is_mobile?` to true
* add a test for Kuwait
* add an exception to the `area_code` presence validation, where it can equal a blank string. Kuwait apparently doesn't have area codes (http://en.wikipedia.org/wiki/Telephone_numbers_in_Kuwait).