Implementation of S3 method for stat.table class

[stat.table](http://www.inside-r.org/packages/cran/Epi/docs/stat.table)
**Basic example**
```
library(Epi) 
### one-way
x <- stat.table(tension,list(count(),mean(breaks)),data=warpbreaks) 
pander(x)
### two-way
y <- stat.table(index=list(tension,wool),mean(breaks),data=warpbreaks) 
pander(y)
```
The numbers become strings here, with a bunch of (probably) unneeded decimals. E.g. from your example:

```r
> pander(y)

--------- ---------------- ----------------
                                *wool*     

*tension*       *A*              *B*       

   *L*    44.5555555555556 28.2222222222222

   *M*           24        28.7777777777778

   *H*    24.5555555555556 18.7777777777778
--------- ---------------- ----------------
```
Probably it would be great to transform the numbers to strings before that with something like:
```
apply(xx, c(1,2), p, wrap = '')
```