Get user_shorten_counts key from data[] in user_shorten_counts method rather than missing shorten_counts key

http://dev.bitly.com/user_metrics.html#v3_user_shorten_counts says the returned json will have the authenticated user's shorten counts listed under "user_shorten_counts", but the current module is looking for them in shorten_counts:

```
>>> c.user_shorten_counts()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "./bitly_api/bitly_api.py", line 289, in user_shorten_counts
    return data["shorten_counts"]
KeyError: 'shorten_counts'
```