title: combine month and day into one date column
body: <p>Using R, I'd like to combine into one column (date) the month nb (month) and the day nb (day) contained in two different columns and use the created column in a date format.</p>
  
  <p>my data frame looks like this:</p>
  
  <pre><code>St Dep Month Day
  A 2 1 1
  B 2 1 1
  B 2 2 1
  A 5 1 1
  A 7 1 1
  C 2 1 1
  C 5 1 1
  </code></pre>
  
  <p>And I want to add a column with something like "Jan-01".</p>