Update the examples for recent bigrquery changes.

PTAL @deflaux (if you'd rather just make this update yourself, I can drop this PR.)

I recently made a change to the signature of the `query_exec` function in
`bigrquery` R package, which meant that all calls in the examples needed
updating. I've done that here, with a few notes/caveats:

 * The previous syntax *forced* you to use the `default_dataset` option, which
   isn't common in most BQ usage. In addition, it looks like all the examples
   have explicit `project:dataset.table` names, which is preferred anyway --
   so I've just dropped the extra arguments. If there's something more subtle
   going on that I didn't catch, let me know.

 * I also renamed `billing_project` to `project` everywhere -- this matches
   both the (updated) `bigrquery` docs and the BigQuery docs more closely.

 * I didn't update the `.md` files -- I did load up one and run it, but it
   looks like there were other changes unrelated to what I was up to that I
   was also picking up. I kept the output, and am happy to include it -- but I
   suspect it makes more sense to do them all at once.

 * My editor removed trailing whitespace in all the files I touched; I can
   revert those chunks if you're worried about `git blame` noise.