title: resample Pandas dataframe and merge strings in column
body: <p>I want to resample a pandas dataframe and apply different functions to different columns. The problem is that I cannot properly process a column with strings. I would like to apply a function that merges the string with a delimiter such as " - ". This is a data example:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  idx = pd.date_range('2017-01-31', '2017-02-03')
  data=list([[1,10,"ok"],[2,20,"merge"],[3,30,"us"]])
  dates=pd.DatetimeIndex(['2017-01-31','2017-02-03','2017-02-03'])
  d=pd.DataFrame(data, index=,columns=list('ABC'))
  
              A   B          C
  2017-01-31  1  10         ok
  2017-02-03  2  20      merge
  2017-02-03  3  30         us 
  </code></pre>
  
  <p>Resampling the numeric columns A and B with a sum and mean aggregator works. Column C however kind of works with sum (but it gets placed on the second place, which might mean that something fails). </p>
  
  <pre><code>d.resample('D').agg({'A': sum, 'B': np.mean, 'C': sum})
  
                A               C     B
  2017-01-31  1.0               a  10.0
  2017-02-01  NaN               0   NaN
  2017-02-02  NaN               0   NaN
  2017-02-03  5.0        merge us  25.0
  </code></pre>
  
  <p>I would like to get this:</p>
  
  <pre><code>...
  2017-02-03  5.0      merge - us  25.0
  </code></pre>
  
  <p>I tried using lambda in different ways but without success (not shown).</p>
  
  <p>If I may ask a second related question: I can do some post processing for this, but how to fill missing cells in different columns with zeros or ""? </p>