title: Create column of value_counts in Pandas dataframe
body: <p>I want to create a count of unique values from one of my Pandas dataframe columns and then add a new column with those counts to my original data frame. I've tried a couple different things. I created a pandas series and then calculated counts with the value_counts method. I tried to merge these values back to my original dataframe, but I the keys that I want to merge on are in the Index(ix/loc).</p>
  <pre><code>Color Value
  Red   100
  Red   150
  Blue  50
  </code></pre>
  <p>I want to return something like:</p>
  <pre><code>Color Value Counts
  Red   100   2
  Red   150   2 
  Blue  50    1
  </code></pre>