Added aggregate example

First of all let me say that I love rxmarbles! It has helped me alot in understanding how rxjs observables work.

I think that aggregate is not simply a clone of scan, since it emits only the final value of the aggregation. I found this extremely useful after doing a groupBy to create another stream where the groups are reduced to single values with accumulated properties:

```
	.groupBy (log) -> 
		log.day
	.flatMap (group) -> 
		group.aggregate (acc, log) -> 
			acc.downtime += log.downtime
			acc
```

Therefore I think this transformation deserves it's own place in the examples.