Fix subset.range for yearmon index

Subsetting would fail when the plotted object has a yearmon index,
because the start/end index values are converted to character as the
3-letter month abbreviation and the 4-digit year. That is not a valid
ISO-8601 range string, as shown in the example below.

    x <- apply.monthly(as.xts(sample_matrix[,4]), last)
    index(x) <- as.yearmon(index(x))
    paste(start(x), end(x), sep = "/")
    [1] "Jan 2007/Jun 2007"

Explicitly format the start/end index values as ISO-8601 before
combining them into the range string.

Fixes #353.

R/plot.R

@@ -742,8 +742,10 @@ addEventLines <- function(events, main="", on=0, lty=1, lwd=1, col=1, ...){
     }
     ypos <- x$Env$ylim[[2*on]][2]*0.995
     # we can add points that are not necessarily at the points on the main series
-    subset.range <- paste(start(xdata[xsubset]),
-                          end(xdata[xsubset]),sep="/")
+    subset.range <-
+      paste(format(start(xdata[xsubset]), "%Y%m%d %H:%M:%OS6"),
+            format(end(xdata[xsubset]), "%Y%m%d %H:%M:%OS6"),
+            sep = "/")
     ta.adj <- merge(n=.xts(1:NROW(xdata[xsubset]),
                            .index(xdata[xsubset]), 
                            tzone=tzone(xdata)),