Enable data import from Google Cloud Storage files

Added two client methods which enable importing data from Google Cloud Storage files, and waiting until the job is complete:

  * client.import_data_from_uris
  * client.wait_for_job

These methods have been tested against live Google Cloud Storage and BigQuery instances, and are currently being used in production.

```python
schema = [ {"name": "username", "type": "string", "mode": "nullable"} ]
job = client.import_data_from_uris( ['gs://mybucket/mydata.json'],
                                    'dataset',
                                    'table',
                                    schema,
                                    source_format=JOB_SOURCE_FORMAT_JSON)

job = client.wait_for_job(job, timeout=60) 
print(job)
```

I see where you're coming from now and agree. I think allowing BigQuery to handle default configurations is the best option (the current approach). I do think we should ensure the docstrings are consistent. To prevent any potential confusion for users, it might be helpful to document that the `None` values will default to Google's configuration, which is always susceptible to being changed.