title: Reshaping data.frame from wide to long format
body: <p>I have some trouble to convert my <code>data.frame</code> from a wide table to a long table.
  At the moment it looks like this:</p>
  
  <pre><code>Code Country        1950    1951    1952    1953    1954
  AFG  Afghanistan    20,249  21,352  22,532  23,557  24,555
  ALB  Albania        8,097   8,986   10,058  11,123  12,246
  </code></pre>
  
  <p>Now I would like to transform this <code>data.frame</code> into a long <code>data.frame</code>.
  Something like this:</p>
  
  <pre><code>Code Country        Year    Value
  AFG  Afghanistan    1950    20,249
  AFG  Afghanistan    1951    21,352
  AFG  Afghanistan    1952    22,532
  AFG  Afghanistan    1953    23,557
  AFG  Afghanistan    1954    24,555
  ALB  Albania        1950    8,097
  ALB  Albania        1951    8,986
  ALB  Albania        1952    10,058
  ALB  Albania        1953    11,123
  ALB  Albania        1954    12,246
  </code></pre>
  
  <p>I have looked at and already tried using the <code>melt()</code> and the <code>reshape()</code> functions
  as some people were suggesting in similar questions.
  However, so far I only get messy results.</p>
  
  <p>If it is possible I would like to do it with the <code>reshape()</code> function since
  it looks a little bit nicer to handle.</p>