Log Message:
Update to 1.7.13

Upstream changes:
Changes in Version 1.7-13

  o The window() method erroneously dropped the dimension in case of
    1-column zoo series. Fixed now.

  o Bug fix in NA handling of rollmax() pointed out by Cory Fletcher.

  o A few as.yearmon/as.yearqtr methods were not registered in
    the NAMESPACE but are now.

  o If there are less then two non-NAs in na.approx() then approx()
    cannot be applied. Instead of throwing an error (as up to version
    1.7-12) simply no NAs are replaced now.

  o Bug fix for lag(z, k = k, na.pad = TRUE) which ignored na.pad = TRUE
    if 'k' was a vector of lags.