Add send_short_code method.

Most of the details are updated in the Readme. Highlights:
* Ability to send messages via Shared Short Code
* Three use cases enabled (`alert`, `2fa`, `marketing`)
* country_code option to allow for future short codes outside the US

Responses are documented as being the same for messages. Short Code message contents are submitted in a pre-approved format (i.e. "Your appointment is tomorrow at ${time}.") and the options (${time}) are passed as `:time => '08:30am'`.