title: Reshape Long to Wide aggregating by 2 variables
body: <p>I would like to reshape my data frame from long to wide by 2 variables (whose combinations create new unique identifier) while aggregating values by product, product2, and date so that the following: </p>
  
  <pre><code>Date           product product2 value
  03/03/2011       a        z        7
  03/03/2011       a        z        2
  03/05/2015       b        z       89
  03/01/2017       a        z        2
  03/03/2017       c        z        6
  </code></pre>
  
  <p>which would yield the following: </p>
  
  <pre><code>      03/03/2011  03/03/2011  03/05/2015  03/01/2017  03/03/2017 
  a z       9                                   2
  b z                               89
  c z                                                        6
  </code></pre>
  
  <p>Should I use dplyr, reshape, reshape2? </p>
  
  <pre><code>df &lt;- structure(list(Date = c("03/03/2011", "03/03/2011", "03/05/2015", "03/01/2017", "03/03/2017"),
               product= c("a", "a", "b", "a", "c"),
               product2= c("z", "z", "z", "z", "z"), 
               value= c(7L, 2L, 89L, 2L, 6L)), 
               .Names= c("Date", "product", "product2", "value"), 
               class= "data.frame", row.names=c(NA, -5L))
  </code></pre>