Introduce BigQuery Regionalization Support

[dwsupplee]

This change set introduces the ability to supply a location when managing jobs.

Some usage examples:

use Google\Cloud\BigQuery\BigQueryClient;

// This default location will be used for creating datasets as well as any job activity
// where we can't infer the location from a network request.
$client = new BigQueryClient([
    'location' => 'asia-northeast1'
]);

// All calls through this job will be routed to the `asia-northeast1` region, using
// the default supplied in the client config.
$job = $client->job('an-existing-job');

// Users can explicitly specify the location if they did not include one in the client
// config or want to override it. With an existing job...
$job = $client->job('an-existing-job', [
    'location' => 'asia-northeast1'
]);

// And when creating a job configuration...
$jobConfig = $client->queryConfig('SELECT * FROM `dataset.table`')
    ->location('asia-northeast1');

foreach ($client->datasets() as $dataset) {
    // `asia-northeast1` is not defaulted here, as network requests have given us
    // the necessary location data and we may be pulling in datasets from other regions. 
    $jobConfig = $dataset->table('my_table')
        ->load('somedata');
}

// Defaults to `asia-northeast1`.
$dataset = $client->createDataset('my_new_dataset');

[dwsupplee]

/cc @jjolma If you would like to take a look as well :)

[jdpedrie]

just a couple little doc things

[dwsupplee]

@tswast Thanks for the solid feedback. PTAL and let me know what you think of the updates.

[tswast]

Much clearer. Thanks!

[jjolma]

LGTM. Thanks!