Added Tagged Method

Added access to Tagged Method. The API is very simple:

```javascript

var tumblr =  require('node-tumblr');

var tags = new tumblr.Tagged(oauth);

tags('nodejs', function(error, posts){
    if(error) throw error;

   console.log(posts);
});


tags({ "tag": "nodejs", "limit": 5 }, function(error, posts){
    if(error) throw error;

   console.log(posts);
});
```