tabs indentation is not considered as invalid

Tab indentation will basically result as a non-char when parsing YAML.

For instance:

``` javascript
a:
        b:
                c: yo mamma
```

As tab indented comes out as:

``` bash
js-yaml test.yaml 

{ a: null, b: null, c: 'yo mamma' }
```

It would be safer and easier to debug if the file would not be parseable on tab indent.
