Restructure and improve "extending choria" section

[khaefeli]

I've tried to start building my own Choria agent, to send some commands to a simple executable service.
The extending Choria section was good, but many important other information was spread across different sections. e.g. like packaging process.

It would be good to have a "getting started" guide for developing:

• Needed / recommended tools (Choria mco, building packages, pdk and other plugins)
• What is the right layout for my module (with agents, applications etc)
• Do I need a client if I've an agent

Additional ideas:

• the code example are somewhere on github as complete module / working code (and not only parts of the code in the how to)
• guide: should I develop an agent, application or use a simple shell command?

[ripienaar]

moved the packaging section in #36