title: rbind two data.frame preserving row order and row names
body: <p>I have a list of <code>data.frame</code> objects which i would like to row append to one another, ie <code>merge(..., all=T)</code>.  However, <code>merge</code> seems to remove the row names which I need to be kept intact. Any ideas? Example:</p>
  
  <pre><code>x = data.frame(a=1:2, b=2:3, c=3:4, d=4:5, row.names=c("row_1", "another_row1"))
  y = data.frame(a=c(10,20), b=c(20,30), c=c(30,40), row.names=c("row_2", "another_row2"))
  &gt; merge(x, y, all=T, sort=F)
       a  b  c  d
    1  1  2  3  4
    2  2  3  4  5
    3 10 20 30 NA
    4 20 30 40 NA
  </code></pre>