GBQ: Handle deprecation of oauth2client.tools.run()
Right now, when you use GbqConnector, you get the following warning:

> WARNING:root:This function, oauth2client.tools.run(), and the use of the gflags library are deprecated and will be removed in a future version of the library.

I'm not sure how quickly this deprecation will happen or if there are alternatives to keep things the way that they are; but at least worthwhile to have an issue here to address it (in the future).

As an aside, I've been glancing at the source for the `bq` client that Google provides as part of Google Cloud SDK and I'm wondering how public the methods are (i.e., if we could adapt some of the code or functions that `bq.py` uses to authenticate and be able to get credentials set up from `gcloud auth login` rather than requiring a separate oauth flow). Not sure though.

Again, cc @jacobschaer , I'd be interested in your opinion on whether this is worth investigating at all. I've been finding `read_gbq` and `to_gbq` quite useful - thanks for putting it together!
