title: split data.frame in two based on one column values
body: <p>Let's suppose I have a data.frame <code>df</code> as follows:</p>
  <pre><code>df=data.frame(one=c(1,2,3,4,5,6,7), 
  two=c('a','a','a','b','b','b','b'), 
  three=c(1123,33,5566,212,1,90,876))
  </code></pre>
  <p>I need to split <code>df</code> in two based on the values of column <code>two</code>, i.e. <code>a</code> and <code>b</code>.</p>
  <p>Here is my desired output:</p>
  <pre><code>  one.x two.x three.x one.y two.y three.y
     1      a    1123     4     b      212 
     2      a      33     5     b        1
     3      a    5566     6     b       90
    NA     NA      NA     7     b      876
  </code></pre>
  <p>Thanks</p>