Read project_id from JSON key file.

[orangain]

A JSON key file provided by Google contains project_id. Now project_id argument of get_client() is optional and read from the JSON key file if json_key or json_key_file is provided. I believe this improve usability of get_client().

Before:

import json
with open('credentials.json') as f:
    credentials = json.load(f)
client = bigquery.get_client(credentials['project_id'], json_key=credentials, readonly=False)

After:

client = bigquery.get_client(json_key_file='credentials.json', readonly=False)

To implement this, I've reverted some codes changed by #91.

[tylertreat]

Thanks!