add two-way binding between parent scope and custom tooltip directive

The original version of $tooltip directive doesn't support add custom variable and function binding between parent scope (e.g. controller) and the custom tooltip directive. Since angular can only add two way binding via element attributes, I add the attributes of original element that binded the custom tooltip (or popover) to the $tooltip template.

Usage:
```html
<!DOCTYPE html>
<html>

<head>
  <script  src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.16/angular.min.js"></script>
  <script  src="angular-ui-bootstrap.js"></script>
  <link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.0.3/css/bootstrap.min.css" />

  <script>
  angular.module('myApp', ['ui.bootstrap'])
  
  .directive('tooltipTestPopup', ['$tooltip', function ($tooltip) {
    return {
      restrict: 'EA',
      replace: true,
      scope: {
        content: '@', 
        placement: '@', 
        animation: '&', 
        isOpen: '&', 
        // bind from controller scope
        customCallback: '&'
      },
      template: 
        "<div class=\"tooltip {{placement}}\" ng-class=\"{ in: isOpen(), fade: animation() }\">\n" +
        "  <div class=\"tooltip-arrow\"></div>\n" +
        "  <div class=\"tooltip-inner\" ng-bind=\"content\" ng-mouseover=\"test()\"></div>\n" +
        "</div>\n" +
        "",
      link: function (scope, elem, attrs) {
        scope.test = function () {

          console.log('call ...');
          scope.customCallback({});
        };
      }
    };
  }])
  
  .directive('tooltipTest', ['$tooltip', function ($tooltip) {
    return $tooltip('tooltipTest', 'tt', 'mouseenter');
  }])
  .controller('Controller', function ($scope) {

      // callback of controller
      $scope.callback = function (msg) {
        console.log('say: ' + msg);
      }
    });
    
  </script>
</head>

<body ng-app="myApp">
  <h1>Two way binding for custom angular-ui-bootstrap tooltip</h1>
  <div ng-contrller="Controller">
    <input type="text" ng-model="msg" value="blablabla" />
    <span tooltip-test="Tooltip ~~~~" tt-custom-callback="callback(msg)">I have a tooltip</span>
  </div>
</body>
</html>
```

The attributes with prefix 'tt-' of `tooltip-test` directive will be mapped to the $tooltip template, so we can bind `custom-callback` directly here.