title: Apply function in summarise using ddply to find percentage of frequencies
body: <p>I have a table similar to this one:</p>
  
  <pre><code>id   name    gender     age      count
  1    apple    Male      13-20      25
  1    apple    Male      21-40      30
  1    apple    Female    13-20      60
  1    apple    Female    21-40      42
  2    banana   Male      13-20      45
  2    banana   Male      21-40      12
  2    banana   Female    13-20      22
  2    banana   Female    21-40      74
  3    orange   Male      13-20      52
  3    orange   Male      21-40      25
  3    orange   Female    13-20      30
  3    orange   Female    21-40      48
  </code></pre>
  
  <p>I want a resulting table with frequency with respect to apple(reference group) like this:</p>
  
  <pre><code>id   gender     banana_wrt_apple   orange_wrt_apple
  1    Male        57/55               77/55
  2    Female      96/102              78/102
  </code></pre>
  
  <p>How can I do this using <code>ddply</code> and <code>summarise</code>?</p>