title: pandas's resample with fill_method: Need to know data from which row was copied?
body: <p>I am trying to use resample method to fill the gaps in timeseries data. But I also want to know which row was used to fill the missed data.</p>
  
  <p>This is my input series.</p>
  
  <pre><code>In [28]: data
  Out[28]: 
  Date
  2002-09-09    233.25
  2002-09-11    233.05
  2002-09-16    230.25
  2002-09-18    230.10
  2002-09-19    230.05
  Name: Price
  </code></pre>
  
  <p>With resample, I will get this</p>
  
  <pre><code>In [29]: data.resample("D", fill_method='bfill')
  Out[29]: 
  Date
  2002-09-09    233.25
  2002-09-10    233.05
  2002-09-11    233.05
  2002-09-12    230.25
  2002-09-13    230.25
  2002-09-14    230.25
  2002-09-15    230.25
  2002-09-16    230.25
  2002-09-17    230.10
  2002-09-18    230.10
  2002-09-19    230.05
  Freq: D
  </code></pre>
  
  <p>I am looking for </p>
  
  <pre><code>Out[29]: 
  Date
  2002-09-09    233.25  2002-09-09
  2002-09-10    233.05  2012-09-11
  2002-09-11    233.05  2012-09-11
  2002-09-12    230.25  2012-09-16
  2002-09-13    230.25  2012-09-16
  2002-09-14    230.25  2012-09-16
  2002-09-15    230.25  2012-09-16
  2002-09-16    230.25  2012-09-16
  2002-09-17    230.10  2012-09-18  
  2002-09-18    230.10  2012-09-18
  2002-09-19    230.05  2012-09-19
  </code></pre>
  
  <p>Any help?</p>