Fixed a majority of pep8 errors in gcloud.

This should be
```python
  - :func:`gcloud.datastore.dataset.Dataset.get_entity`
    to retrieve an existing entity.
```
which yields


![should_be](https://cloud.githubusercontent.com/assets/520669/4468570/04d55ea2-4900-11e4-94e4-3afc0da9b4ac.png)

while the current implementation yields


![curr_state](https://cloud.githubusercontent.com/assets/520669/4468573/0b8e58d4-4900-11e4-8e0a-f3b9f7864fbe.png)