Charm execution graph in docs

[Paul Eipper]

Hi all,

This is a documentation request for having a life cycle overview of a
charm, linking the events in a way so that the complete set of
possible states is visible and what are their triggers.

For reference, I created a graph and shared it on stackoverflow some time ago:
http://stackoverflow.com/a/25980368/324731

Would be best to have an official source though, updated to latest changes.

att,
--
Paul Eipper