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)
```

In case you're looking at this in email, I modified the previous comment slightly, so please take a look on GitHub.

I think (2) is definitely out of the running, so we need to choose between (1) and (3). 

I noticed that BigQuery failed silently when various combinations of non-CSV default values were provided.  I never quite figured out which combinations were doing this, and managed to get the service working only when I reverted to a minimal load configuration, so that's why I went with the current approach.  (Though, it's possible that this was due to some other problem in my early code, so I could go back and look at that.)