extra::stats::write_boxplot takes positive sample values only

The program

``` rust
extern mod extra;

use extra::stats;

fn main() {
    let s = stats::Summary::new([0.0, 2.0, 3.0]);
    stats::write_boxplot(std::io::stdout(), &s, 100) 
} 
```

outputs 

```
NaN |[#]                                                                                          | 3
```

because the choice of upper and lower limits of the boxplot involves `log10()`.
