Refactor key checking
Refactor key checking

1. Rename `Twarc.check_keys` to `Twarc.get_keys` to make the
  usage clear.
2. Add a prototype `Twarc.validate_keys`, that will check that
  the full set of keys is present and valid against the Twitter
  API.

DocNow/twarc,remagio/twarc,edsu/twarc,hugovk/twarc,remagio/twarc