title: pandas replace multiple values one column
body: <p>In a column risklevels I want to replace Small with 1, Medium with 5 and High with 15.
  I tried:</p>
  <pre><code>dfm.replace({'risk':{'Small': '1'}},
              {'risk':{'Medium': '5'}},
              {'risk':{'High': '15'}})
  </code></pre>
  <p>But only the medium were replaced.
  What is wrong ?</p>