Implement absolute event iterator.

[PieterPenninckx]

Add an iterator that allows to iterate over the events with time relative to the start of the track, rather than to the previous event.

[PieterPenninckx]

This pull request is not really needed since one can do something like the following:

let mut offset = 0;
let iter = track.iter().map(move |e| {
    offset += e.delta.as_int() as u64;
    (offset, e.kind)
});