Implementation of S3 method for coxph class

S3 method for coxph class (irregular time series)

Basic example:
```
library(survival)
test1 <- list(time=c(4,3,1,1,2,2,3), 
              status=c(1,1,1,0,1,1,0), 
              x=c(0,2,1,1,1,0,0), 
              sex=c(0,0,0,0,1,1,1)) 
x <- coxph(Surv(time, status) ~ x + strata(sex), test1)
pander(x)
x1 <- coxph(Surv(stop, event) ~ (rx + size + number) * strata(enum) + 
              cluster(id), bladder1)
pander(x1)
```