title: Fill date and id gaps, then lag in pandas
body: <p>I have a dataframe, df1, which consists of entities across time and their corresponding value.  df1 looks like this:</p>
  
  <pre><code>Index        Date    ID   Values           
     0     2016-11-15   1      3
     1     2016-11-16   1      5
     2     2016-11-16   2      6
     3     2016-11-17   2      7            
     4     2016-11-18   2      2   
  </code></pre>
  
  <p>Obviously, there are gaps in the dates for both IDs.<br>
  I would first like to fill the date and ID gaps so it looks like this:</p>
  
  <pre><code> Index        Date   ID   Values           
     0     2016-11-15   1      3
     1     2016-11-16   1      5
     2     2016-11-17   1      0
     3     2016-11-18   1      0
     4     2016-11-15   2      0
     5     2016-11-16   2      6
     6     2016-11-17   2      7            
     7     2016-11-18   2      2    
  </code></pre>
  
  <p>Then, I want to create a lagged column for "Values" so it uses the previous day's value for each id:</p>
  
  <pre><code> Index        Date   ID   Values  Lagged Values          
     0     2016-11-15   1      3         Nan
     1     2016-11-16   1      5          3
     2     2016-11-17   1      0          5
     3     2016-11-18   1      0          0
     4     2016-11-15   2      0         Nan
     5     2016-11-16   2      6          0
     6     2016-11-17   2      7          6 
     7     2016-11-18   2      2          7
  </code></pre>
  
  <p>Thanks!</p>