Pubsub surface uses long imports

In the pubsub surface docs page, we use long-style imports instead of the shortcut ones, that is:

``` python
from gcloud.pubsub.topic import Topic
topic = Topic('topic_name')
```

instead of

``` python
from gcloud import pubsub
topic = pubsub.Topic('topic_name')
```

or 

``` python
from gcloud.pubsub import Topic
topic = Topic('topic_name')
```
