ENH: improve bigquery connector to optionally allow use of gcloud credentials


The pandas google bigquery connector can be difficult to use from an ipython notebook: when no authentication token exists, it fails silently (and can hang the notebook).  This happens because the oauth2client library is trying to bring up a browser on the console, to request an auth token.  

This PR provides an improved mechanism: an optional `use_gcloud_auth` argument which specifies that the user wants to employ a pre-existing authentication token, generated using [gcloud auth login](https://cloud.google.com/sdk/gcloud/reference/auth/login).  An error is raised if the credentials file is missing.
@ichuang Can we have those imports moved to the top to conform with PEP8?