title: summarize groups into intervals using dplyr
body: <p>H,
  I have a data frame like this:</p>
  
  <pre><code>d &lt;- data.frame(v1=seq(0,9.9,0.1),
              v2=rnorm(100),
              v3=rnorm(100))
  
  &gt; head(d)
     v1          v2         v3
  1 0.0 -0.01431916 -0.5005415
  2 0.1 -1.01575590  1.5307473
  3 0.2  1.00081065 -0.1730830
  4 0.3 -1.20697918  0.5105118
  5 0.4 -2.16698578 -1.0120544
  6 0.5  0.33886508  0.4797016
  </code></pre>
  
  <p>I now want a new data frame that summarizes all values in the intervals 0-0.99, 1-1.99, 2-2.99, 3-3.99,.... by the mean for example</p>
  
  <p>like this</p>
  
  <pre><code>start end mean.v2 mean.v3
      0   1     0.2     0.1
      1   2     0.5     0.4
  </code></pre>
  
  <p>and so on</p>
  
  <p>thanks</p>
  
  <p><em>Update</em> I should add that in my real data set the observations in each interval are of different lengths and they don't always start at zero or end at 10</p>