title: Merge two DataFrames based on multiple keys in pandas
body: <p>Does pandas (or another module) have any functions to support merge (or join) two tables based on multiple keys?</p>
  
  <p>For example, I have two tables (DataFrames) <code>a</code> and <code>b</code>:</p>
  
  <pre><code>&gt;&gt;&gt; a
  A  B  value1
  1  1      23
  1  2      34
  2  1    2342
  2  2     333
  
  &gt;&gt;&gt; b
  A  B  value2
  1  1    0.10
  1  2    0.20
  2  1    0.13
  2  2    0.33
  </code></pre>
  
  <p>The desired result is:</p>
  
  <pre><code>A  B  value1  value2
  1  1      23    0.10
  1  2      34    0.20
  2  1    2342    0.13
  2  2     333    0.33
  </code></pre>