MCO-484 restructure 'Writing SimpleRPC Agents' for 2.6.0

With this commit we rewrite the demonstration agent so it works directly
against MCollective 2.6.

We also remove the references to older version 2.x behaviour, and
restructure the document in an attempt to tour the moving parts of agents and
actions more naturally.

New Structure:
```
* Conventions regarding Incoming Data
** Sample Agent
*** Agent Naming
** The DDL
** Actions
*** Declaring in the DDL
*** Implementing in Ruby
**** Accessing Input
**** Running Shell Commands
*** Implementing with external_action
** Constructing Replies
*** Reply Data
*** Reply Status
** Sharing code between agents
** Authorization
** Auditing
** Logging
** Data Caching
** Processing Hooks
** Agent Configuration
** Agent Activation
```