Extra Sensor now called Special Sensor

Consistency was necessary as the type names are used to match
sensor counts with the sensor details.
Add to_hash() and get()

to_hash: Convert whole object to a hashref

get: Retrieve the value of one named key.
Finalise the /devices endpoint

/devices        ... returns a hashref of available device types, and counts of each

/devices/:type  ... returns details of each registered device of that type.
Start to document the REST interface