title: How to apply string methods to multiple columns of a dataframe
body: <p>I have a dataframe with multiple string columns.  I want to use a string method that is valid for a series on multiple columns of the dataframe.  Something like this is what I would wish for:</p>
  
  <pre><code>df = pd.DataFrame({'A': ['123f', '456f'], 'B': ['789f', '901f']})
  df
  
  Out[15]: 
        A     B
  0  123f  789f
  1  456f  901f
  
  df = df.str.rstrip('f')
  df
  Out[16]: 
       A    B
  0  123  789
  1  456  901
  </code></pre>
  
  <p>Obviously, this doesn't work because str operations are only valid on pandas Series objects.  What is the appropriate/most pandas-y method to do this?</p>