pandas.io.gbq Version 2

closes #5840 (as new interface makes it obsolete)
closes #6096 

@jreback : We still have some documentation to work on, but we would like your initial thoughts on what we have so far. The key change for this version is the removal of bq.py as a dependency (except as a setup method for a test case). Instead, we rely entirely on the BigQuery python API. We also simplified to_gbq() significantly. Though it cost a few features, the code is much more manageable and less error prone (thanks Google!). Test cases are much more granular and run significantly faster. To use the test cases fully, a BigQuery project_id is still required, though there are some unittests offline. 
Jordan works for Google, so maybe he could verify that they changed the functionality on SELECT *, but it definitely has not always returned the columns in the table order which is why we  added it. Part of the reason we have labeled this as "experimental" is that we have experienced several changes on the Google side and a few bugs over the past year, just FYI.

If the behavior has indeed changed, then there is no reason to have it. It is fairly easy to remove.