Add etag and if-modified-since support. (http request)

The execMethod() returns a third parameter which is the http response.
(In order to get the date and the etag header values)

lib/moviedb.js

@@ -40,8 +40,8 @@ MovieDB.prototype.requestToken = function(fn){
     if(err) {
       fn(err);
     } else {
-       self.token = res.body;
-       fn();
+    self.token = res.body;
+    fn();
     }
   });
 
@@ -85,6 +85,17 @@ var execMethod = function(type, params, endpoint, fn){
   var req = request(type, endpoints.base_url + endpoint)
             .query({api_key : this.api_key})
             .set('Accept', 'application/json');
+  
+  if (params.ifNoneMatch) {
+    req=req.set('If-None-Match', params.ifNoneMatch);
+    
+  } else if (params.ifModifiedSince) {
+    var t=params.ifModifiedSince;
+    if (t.toUTCString) {
+      t=t.toUTCString();
+    }
+    req=req.set('If-Modified-Since', t);
+  }
 
   if(type === 'GET')
     req.query(params);
@@ -93,9 +104,9 @@ var execMethod = function(type, params, endpoint, fn){
 
   req.end(function(err, res){
     if(err){
-      fn(err);
+      fn(err, null, res);
     } else {
-      fn(null, res.body);
+      fn(null, res.body, res);
     }
   });
 };