Animation sequence support

[jheer]

Add abstraction to simplify staged animations? The abstraction would take a set of transitions (wrapped in function instances) and then oversee their sequential execution. It should enable easy parameterization of stage durations, be cancelable, etc...

Two potential cases:

1. Independent transitions: no element-level chaining. Specify function list and list of durations (or offsets) and run each stage accordingly.
2. Dependent transitions: transitions are chained per-element (e.g., .each("end", next()), allowing staggering to propagate. The transition wrapper function might take a "next" parameter in addition to a duration to avoid hard-wiring the next transition.

[mbostock]

I think I'm happy with the level of support we have now, which includes transition.transition() and transition.each("end", callback). We might consider adding a transition.transition("end"), leaving the default as transition.transition("start").