Add method for sending SMS texts

This method currently uses the undocumented API endpoint /v2/ephemerals to send SMS
messages from a device specified by its 'device_iden' to a phone number.

Obviously, not every device can send SMS texts. Devices that can have the property 'has_sms'
set to true in their descriptions. This can be checked with PushBullet::getDevices().

Beware of undocumented behavior and meaningless responses from the API.