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)
```

Hi Tyler - You are right - there's an inconsistency between the docstring and the function defaults.  That at a minimum needs to change.

Here's the problem I ran into:  not all of the defaults may be set under all conditions.  For example, if you set source_format=JSON, providing the default values for allow_jagged_rows, allow_quoted_newlines, field_delimiter, quote and skip_leading_rows causes BigQuery to reject the job.  

To be consistent, I changed all optional values to None, which only get set if the user explicitly provides a value.  We have a couple of options:

1. All optionals are None (current approach)
Pro: * Consistent
        * Specifies a minimal load configuration, as per examples provided by Google.
        * Defaults are always as defined by BigQuery
Con: 
       * Nasty bug if Google changes BigQuery defaults 

2. Set all values to default
  Pro:
    * Consistent
    * No bug if Google changes BQ defaults

3. Provide explicit values for non-CSV source_format variables
    Pro:
       * partially resistant to service-side configuration default changes
    Con:
       * Inconsistent treatment of default values

I also 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 (1).