make print.duration handle vectors and data frames

[garrettgman]

currently:

a
1 month and 1 second

c(a,a)
$months
[1] 1

$seconds
[1] 1

$months
[1] 1

$seconds
[1] 1

list(a,a)
[[1]]
1 month and 1 second

[[2]]
1 month and 1 second

data.frame( one = a)
one.months one.seconds
1 1 1

any suggestions?

[hadley]

I think I've mostly fixed this. If we really want to put durations in a data frame, that should be a separate issue.