Add pair_sms

Tidying up the language library. Matching functionality of toopher-python library. 
Should we include an options hash for extensibility? I believe it would look something like this:

```ruby
def pair_sms(phone_number, user_name, phone_country='', options={})
  parameters = {
    'phone_number' => phone_number,
    'user_name' => user_name
  }
  phone_country.empty? or (parameters['phone_country'] = phone_country)
  parameters.merge(options)
  return PairingStatus.new(post('pairings/create/sms', parameters))
end
end
```