Added support to restrict server side routes by HTTP method

Added optional second parameter to Meteor.Router.add on server side to specify the HTTP method which the route should be called with. Example:

``` javascript
Meteor.Router.add('/posts/:id.xml', 'GET', function(id) {
  return constructXMLForId(Posts.findOne(id));
});
Meteor.Router.add('/posts/:id.xml', 'DELETE', function(id) {
  Posts.remove(id);
  return [204, 'No Content'];
});
```

The implementation is completely backwards compatible, so if you don't specify an HTTP method, the route will still be accessible with all HTTP methods.