fechar div clicando fora

<script type="text/javascript">
var visivel = false;
function mostrarOculto1(){
var objDiv1 = document.getElementById('log_reg');
var objDiv2 = document.getElementById('tudo');
   if (visivel == false){
                objDiv1.style.display = "block";
  objDiv1.style.zIndex =1000;
  objDiv1.style.position = "absolute";
  objDiv2.style.marginTop ="70px";
                visivel = true;

   }else{
        objDiv1.style.position = "absolute";
                  objDiv1.style.display = "none";
        objDiv2.style.marginTop ="30px";
                  visivel = false;                     
}
}</script>

<a onclick="mostrarOculto1();">Login</a>

<div id="log_reg">
        <div id="formulario">
          <form name="form1" id="form1" action="" method="post">
                <fieldset>
                  <label> <span>Nickname:</span>
                        <input type="text" name="nickname" id="nickname" />
                  </label>
                  <label> <span>Password:</span>
                        <input type="password" id="password" name="password" />
                  </label>
                  <input type="submit" value="login" id="login" name="login" class="send"/>
                </fieldset>
          </form>
        </div>
  </div>

body {
background-color: #44c8cc;
background-image: url(images/fundo.png);
background-repeat: repeat-x;
background-attachment: fixed;
background-position: bottom;
}

#log_reg{
width: 100%;
margin-top: 0px;
height:40px;
background-color:#039;
display:none;

}