title: pair<int,int> pair as key of unordered_map issue
body: <p>My code:</p>
  
  <pre><code> typedef pair&lt;int,int&gt; Pair
    tr1::unordered_map&lt;Pair,bool&gt; h;
    h.insert(make_pair(Pair(0,0),true));
  </code></pre>
  
  <p>Erorr</p>
  
  <pre><code> undefined reference to `std::tr1::hash&lt;std::pair&lt;int, int&gt; &gt;::operator()(std::pair&lt;int, int&gt;) const'
  </code></pre>
  
  <p>Something I need to fix?</p>
  
  <p>thanks</p>