Add pair_sms

Tidying up the language library. Matching functionality of toopher-python library. 
Ruby doesn't have `**kwargs` -- the equivalent for Ruby 1.9 is the `options` hash ((Ruby 2.0 added better support for keyword args)[http://robots.thoughtbot.com/ruby-2-keyword-arguments]). We've also used the `options` hash in the `pair` method above `pair_sms`.