title: TS Interface doesn't force functions signature on implementers
body: <pre><code>interface test{
      foo(boo:string);
  }
  class coo implements test{
      foo(){
  
      }
  }
  </code></pre>
  
  <p>In <a href="http://www.typescriptlang.org/Playground/">playGround</a>
  this doesn't generate and error although the function signature is not as the interface
  says, the expected behavior of interface is to force the signature..</p>
  
  <p>why is this behavior? </p>
  
  <p>Thanks</p>