Implementation of S3 method for smooth.spline class

[smooth.spline](http://stat.ethz.ch/R-manual/R-patched/library/stats/html/smooth.spline.html)
**Basic example**
```
require(graphics)
attach(cars)
plot(speed, dist, main = "data(cars)  &  smoothing splines")
cars.spl <- smooth.spline(speed, dist)
pander(cars.spl)
```