BigQuery: Add default location to Client

It is potentially quite cumbersome to pass in a location parameter to nearly every API request method. It would be nice to set a default location when you create a client to be used when creating datasets or jobs.

I imagine the number of defaults we'd want to set will grow (encryption configuration, maximum bytes billed, etc.) but location is basically a peer to project, so it makes sense to me to handle it in the same way.