Wraps decorator functions (fix #44).

@codingjester I actually ran into this using this lib today. This should fix it.

#### Before

Before, when running:

```py
help(pytumblr.TumblrRestClient.avatar)
```

We got

![screen shot 2014-09-23 at 01 17 48](https://cloud.githubusercontent.com/assets/97697/4367212/d8fc6f62-42d0-11e4-9455-3f793c03e7c0.png)


#### After

Now, when doing:

```py
help(pytumblr.TumblrRestClient.avatar)
```

We get:

![screen shot 2014-09-23 at 01 17 21](https://cloud.githubusercontent.com/assets/97697/4367214/e3bb00c6-42d0-11e4-81bc-8051b3bd2664.png)