Update `bigrquery` for new `httr` changes.

This includes the following updates:

* switch from `RJSONIO` to `jsonlite`.

* deprecate `get_sig`

* change top-level functions in `request.r` to take a token instead of a
signature (which is necessary to be able to refresh the token as needed).

Fixes #6, #8, and #10.
code review feedback
Merge pull request #11 from craigcitro/new_httr

Update `bigrquery` for new `httr` changes.