title: numpy: broadcast matrix multiply accross array
body: <p>I have a <code>3xN</code> array, conceptually an array of <code>N</code> 3-vectors, I want to construct the 
  array which results from matrix multiplying a given <code>3x3</code> matrix with each column of the 
  array.  Is there a good way to do this in a vectorized manner?</p>
  
  <p>Currently, my problem is <code>3xN</code>, but I may need to consider <code>3xNxM</code> (or more) in the future.</p>
  
  <p>Loopy approach</p>
  
  <pre><code>U=numpy.rand( [3,24] )
  
  R=numpy.eye(3) # placeholder
  
  for i in xrange( U.shape[1]):
      U[:,i]=numpy.dot( R, U[:,i] )
  </code></pre>