Fix futures and options expiry functions

It's not clear how long these functions have been broken. The source of
the break started in 2008, when Jeff removed the generics and methods
for years(), quarters(), months(), etc. This might have been okay for
awhile, but perhaps S3 dispatch has become more restrictive since then
(e.g. a guess: methods in packages must be registered, not just named
like a S3 method).

The fix appears to be simple: use .indexmon() to replace the defunct
months.xts() method that isn't registered. Also delete the months.xts()
method to avoid more confusion.

Fixes #257.

R/months.R

@@ -11,11 +11,7 @@ function(x) {
   which(format(index(x),"%d") > 14 &
         format(index(x),"%d") < 22 &
         format(index(x),"%w")==5 &
-        as.numeric(months(x,TRUE)) %in% c(3,6,9,12))
-}
-months.xts <- function(x, abbreviate)
-{
-  months(structure(.index(x), class=c('POSIXt','POSIXct'))) 
+        (.indexmon(x)+1) %in% c(3,6,9,12))
 }
 
 `nmicroseconds` <-