Can't dump circular data

```
> cat circular.js 
x = {};
x.foo = x;
require('js-yaml').dump(x);
> node circular.js 

/home/ingy/src/node_modules/js-yaml/lib/js-yaml/type/timestamp.js:25
match = YAML_TIMESTAMP_REGEXP.exec(object);
                                ^
RangeError: Maximum call stack size exceeded
> 
```

This is not a contrived use case. I got this trying to dump an http request object.
