title: Reshape a dataframe with reshape2
body: <p>I have this data.frame:</p>
  
  <pre><code>habitat &lt;- data.frame(
      proportion = c(0.053, 0.139, 0.050, 0.756, 0.001, 0.084, 0.000, 0.011, 0.005, 0.066, 0.810, 0.025), 
      habitat = c("Non-irrigated arable land", "Pastures", "Natural grasslands", "Intertidal flats", "Sea and ocean", "Non-irrigated arable land", "Pastures", "Natural grasslands", "Beaches, dunes, sands", "Salt marshes", "Intertidal flats", "Sea and ocean"), 
      zone = c(rep("dark", 5), rep("light", 7)))
  
  habitat
  
     proportion                   habitat  zone
  1       0.053 Non-irrigated arable land  dark
  2       0.139                  Pastures  dark
  3       0.050        Natural grasslands  dark
  4       0.756          Intertidal flats  dark
  5       0.001             Sea and ocean  dark
  6       0.084 Non-irrigated arable land light
  7       0.000                  Pastures light
  8       0.011        Natural grasslands light
  9       0.005     Beaches, dunes, sands light
  10      0.066              Salt marshes light
  11      0.810          Intertidal flats light
  12      0.025             Sea and ocean light
  </code></pre>
  
  <p>I need it reshaped to look like this:</p>
  
  <p><img src="https://i.stack.imgur.com/RcQag.jpg" alt="enter image description here"></p>
  
  <p>Can this be done with reshape2?</p>