function addInputSubmitEvent(form, input){
   input.onkeydown = function(e){
      e = e || window.event;
      if(e.keyCode == 13){
         form.submit();
         return false;
      }
   }
}
window.onload = function(){
   var forms = document.getElementsByTagName('form');
   for(var i = 0; i < forms.length; i++){
      var inputs = forms[i].getElementsByTagName('input');
      for(var j = 0; j < inputs.length; j++)
         addInputSubmitEvent(forms[i], inputs[j]);
   }
}