cbind() time series without NAs
===

I've observed that for many operators on overlapping time series, the result is given only for the overlapping portion, which is nice:

    > (ts1 <- ts(1:5, start=1, freq=3))
    Time Series:
    Start = c(1, 1) 
    End = c(2, 2) 
    Frequency = 3 
    [1] 1 2 3 4 5
    > (ts2 <- ts((7:3)^2, start=2, freq=3))
    Time Series:
    Start = c(2, 1) 
    End = c(3, 2) 
    Frequency = 3 
    [1] 49 36 25 16  9
    > ts1 + ts2
    Time Series:
    Start = c(2, 1) 
    End = c(2, 2) 
    Frequency = 3 
    [1] 53 41

However, this doesn't seem to be the case with `cbind()`.  While the output is aligned properly, `NA`s are created for the non-overlapping data:

    > (mts <- cbind(ts1, ts2))
    Time Series:
    Start = c(1, 1) 
    End = c(3, 2) 
    Frequency = 3 
             ts1 ts2
    1.000000   1  NA
    1.333333   2  NA
    1.666667   3  NA
    2.000000   4  49
    2.333333   5  36
    2.666667  NA  25
    3.000000  NA  16
    3.333333  NA   9

Is there a way to perform that `cbind()` without creating the rows with `NA` in them?  Or if not, what's a good way to take the result and strip off the rows with the `NA`s?  It's not a simple matter of subscripting, because then it loses its timeseries nature:

    > mts[complete.cases(mts),]
         ts1 ts2
    [1,]   4  49
    [2,]   5  36

Maybe something with `window()`, but calculating the start & end times for the window seems a little yucky.  Any advice is welcome.

