Support optional arguments to Observable.log

if you're ever logging more than one observable, it can be difficult to know which values come from which observable. this change allows you to write,

```coffeescript
Bacon.sequentially(100, [1,2,4]).log("streamA")
Bacon.sequentially(100, [3,2,1]).log("streamB")
```

for a console output that looks like,

```
streamA 1
streamB 3
streamA 2
streamB 2
streamA 4
streamA <end>
streamB 1
streamB <end>
```