Log Message:
Update to 1.8.0

Upstream changes:
Changes in Version 1.8-0

  o zoo() and zooreg() gained a calendar = getOption("zoo.calendar", TRUE)
    argument so that, by default, yearqtr and yearmon are used as the time
    index for regular zoo series with frequency 4 and 12, respectively. With
    calendar = FALSE the behavior from previous versions with plain
    unclassed time indexes can be obtained. Set options(zoo.calendar = FALSE)
    to always retain the old behavior.

  o na.locf() is made faster by using a diff()-based solution rather than
    cumsum() (suggested by Ruben Arslan). Also, the workhorse function
    na.locf0() underlying the default na.locf() method is now exported
    in the user interface as it is faster (but also supports less options).

  o Better support of vector-valued 'split' arguments in read.zoo().

  o Special-cased rollmax(..., k = 1) to return the input coredata().

  o as.character() for "yearqtr" objects did not process NA elements \ 
correctly.