Clarify how a graphic should handle action method calls

[nytamin]

To be discussed in the Working Group

[nytamin]

Notes from discussion today:
We should not put too much wording into the behavior of the Graphic, rather tilt it towards what the graphic can expect in terms of the timing of it's actions.

If a Promise is not resolved within a certain amount of time, the caller MAY consider the promise as failed.
...
The Graphic can expect each call to its actions to have waited for its previous action Promise to have resolved.
If a call to an action comes before the Promise has resolved, the graphic is free to handle that as it sees fit.
A controller that doesn't wait for the promises, cannot make any assumptions of the resulting the behavior.

[dvdeurse]

I merged #34, which basically contains the same information (amongst others) as was proposed in this pull request. So I suggest to close this one without merging.