Added missing message types

[rob42]

Added subscribe, unsubscribe, get, put, list message types

[rob42]

Aargh..this pull has inherited #230 which has not yet been merged...

[rob42]

Merging as there are no comments or objections

[rob42]

Re subscriptions - oops - should be a $ref. I will fix the duplication.

The need for the get and put over websockets came from my recent work on freeboard-sk and xmpp. When using an async message transfer (like xmpp or any pub/sub protocol) a REST call doesnt work anymore. It becomes necessary to store and forward the message, but it was not valid (or defined anywhere else) in the current delta.json.

The get has no equivalent except to subscribe and get value, and unsubscribe for devices without http capabilities. The put could be done with an update but the use case is different, so separating them allows for finer-grained control. list is not really as important, I included it for completeness.

I dont want to remove get or put as I use these now.
I will add to the docs for these.

[rob42]

See #239