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)
```