Log Message:
math/R-zoo: Update to 1.8.1

Changes:

  o New .DollarNames method (contributed by Josh Ulrich) to enable tab
    completion when using zoo_object$...

  o New default method for zoo::as.Date() that simply dispatches to
    base::as.Date().

  o rollapply(x, k, FUN, align = "right") lead to an error if \ 
length(x) < k.
    This has been corrected to match the behavior of the other align types.

  o rollmean(), rollsum(), rollmedian(), and rollmax() no longer throw an
    error if the series is shorter than the window size. Instead rollapply()
    is called which either returns an empty series (if "fill" is not \ 
specified)
    or a padded series (if "fill" is specified, e.g., to NA).

  o read.zoo() now also handles "tbl" objects by coercing them to proper
    "data.frame"s first. Also gained a read = read.table argument to \ 
simplify
    convenience interfaces read.csv.zoo() etc.

  o The fortify() method gained a "names" argument so that the column names
    of the (melted) series can be modified.