title: Detect the first change in a Pandas dataframe
body: <p>I facing the issue that I have an Pandas dataframe that saves the status of a variable V1 every day. I am only interested in the information when does the variable changes its status and what is the new status. </p>
  
  <pre><code>test_dataframe = pd.DataFrame()
  test_dataframe['V1'] = ['X','Y','X','X','Y','X','Y','X']
  test_dataframe['Status'] = ['A','C','B','B','D','B','D','A']
  test_dataframe['Date'] = [pd.to_datetime('2017-1-1'),pd.to_datetime('2017-1-2'),pd.to_datetime('2017-1-3'),pd.to_datetime('2017-1-4'),pd.to_datetime('2017-1-5'),pd.to_datetime('2017-1-6'),pd.to_datetime('2017-1-7'),pd.to_datetime('2017-1-8')]
  print(test_dataframe)
  </code></pre>
  
  <p>Leading to following dataframe </p>
  
  <pre><code>  V1 Status       Date
  0  X      A 2017-01-01
  1  Y      C 2017-01-02
  2  X      B 2017-01-03
  3  X      B 2017-01-04
  4  Y      D 2017-01-05
  5  X      B 2017-01-06
  6  Y      D 2017-01-07
  7  X      A 2017-01-08
  </code></pre>
  
  <p>I am interested in: When does the variable change its status, and what is the new status?</p>
  
  <p>The result should be:</p>
  
  <pre><code>V1 Status Date
  -----------------
  X  A      date_1
  X  B      date_3
  X  A      date_8
  Y  C      date_2
  Y  D      date_5
  </code></pre>
  
  <p>Can someone help?
  THX</p>