Add global_phone validator

Validate global phone numbers in Rails 3.x and ActiveModel.

```ruby
class Person < ActiveRecord::Base
  validates :home_phone, :global_phone => true
end

Person.new(home_phone: '+61 3 9876 0010').valid?
# => true

class User < ActiveRecord::Base
  validates :work_phone, :global_phone => { :using => :work_country_code }
  
  def work_country_code
    'AU'
  end
end

User.new(work_phone: '03 9876 0010').valid?
# => true
```