Implementation of S3 method for survdiff class

[Survdiff class](http://stat.ethz.ch/R-manual/R-devel/library/survival/html/survdiff.html)
**Basic examples**
```
library(survival)
x <- survdiff(Surv(futime, fustat) ~ rx,data=ovarian)
pander(x)
y <- survdiff(Surv(time, status) ~ pat.karno + strata(inst), data=lung)
pander(y)
expect <- survexp(futime ~ ratetable(age=(accept.dt - birth.dt),
                                     sex=1,year=accept.dt,race="white"), jasa, cohort=FALSE,
                  ratetable=survexp.usr)
## actual survival is much worse (no surprise)
z <- survdiff(Surv(jasa$futime, jasa$fustat) ~ offset(expect))
pander(z)
```