Box class: Read-only property-getters for offset, root, raw and parent fields

[tchakabam]

@tieleman hey, i'd find this kind of functions useful, what do you think?

I reconned that there is a problem with the general design of your box class. It would be better if the read iso fields would be in a props member-object that holds all the values like a dictionnary instead of using the main object as this dictionnary. that will allow to implement utility methods like these on top of the main class.

[tchakabam]

(and obviously i would write some tests if this is seen as a useful feature)

[tieleman]

@tchakabam: Yes, I can see the usefulness of this. I'm currently a bit swamped in work, but tests would be nice yes.