Implementation of S3 method for clogit class

[clogit](http://stat.ethz.ch/R-manual/R-devel/library/survival/html/clogit.html)
Also fixed small error for coxph class (information was not complete)
**Basic example**
```
resp <- levels(logan$occupation)
n <- nrow(logan)
indx <- rep(1:n, length(resp))
logan2 <- data.frame(logan[indx,],
                     id = indx,
                     tocc = factor(rep(resp, each=n)))
logan2$case <- (logan2$occupation == logan2$tocc)
x <- clogit(case ~ tocc + tocc:education + strata(id), logan2)
pander(x)
```