Support UTF-8 in Google BigQuery results
Given that the entire Google BigQuery API returns UTF-8, it would make sense to handle UTF-8 output from BigQuery in the `gbq.read_gbq` IO module.

I'd love to do a pull request but I'm not sure the preferred way of handling this. I'd assume that [this line](https://github.com/pydata/pandas/blob/master/pandas/io/gbq.py#L123) should be changed to `field_value = field_value.decode('utf-8')`. I made that change and tests passed but figuring out how to properly test UTF-8 encoding is giving me some trouble (keeps making my test addition fail).
