Refactor GoogleCredentials.get_application_default

This should have a diffbase of #83 but we can't specify those on GitHub.

As in #83 (after #82), I prefer to rebase this after #83 is merged.
The idea is that `_implicit_credentials_from_files` would actually handle the 3LO (in `checkers` below).

This is done in #80, but it is super hacky because the files are in a different format than the JSON files used by `flow_from_clientsecrets`. This is due to inheriting the `gcloud` format. I also had to hack around `run_flow` requiring storage, when none was needed.