Implementation of S3 method for zoo class

Implementation of S3 method for zoo class
I'm not sure if to format dates with panderOption date
```
colnames(c.tab) <- trunc(time(x))
```
or
```
colnames(c.tab) <- format(trunc(time(x)), panderOptions('date'))
```

**Basic examples**:
```
x.Date <- as.Date("2003-02-01") + c(1, 3, 7, 9, 14) - 1
x <- zoo(rnorm(5), x.Date)
pander(x)
z <- zoo(cbind(foo = rnorm(5), bar = rnorm(5)))
pander(z)
```