title: pandas count values in each column of a dataframe
body: <p>i'm lookng to find a way to count the number of values in a column and its proving trickier than i originally thought.</p>
  
  <pre><code>       Percentile   Percentile1 Percentile2 Percentile3
  0       mediocre    contender   contender   mediocre
  69      mediocre    bad         mediocre    mediocre
  117     mediocre    mediocre    mediocre    mediocre
  144     mediocre    none        mediocre    contender
  171     mediocre    mediocre    contender   mediocre
  </code></pre>
  
  <p>i'm trying to create something looking like the following output.  It takes the four options and counts them per column.  It is essentially a pd.value.counts for each column.  Any help would definitely be appreciated.</p>
  
  <pre><code>         Percentile     Percentile1     Percentile2     Percentile3
  mediocre:    5               2               3               4
  contender:   0               1               2               1
  bad:         0               1               0               0
  none:        0               1               0               0
  </code></pre>