Add to/from json methods to Credentials classes

[dlorenc]

This adds to_json and from_json methods to GoogleCredentials
and _ServiceAccountCredentials classes. This allows them to
be serialized by multistore_file.

Resovles: #384

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[googlebot]

CLAs look good, thanks!

[dlorenc]

Thanks for the review, addressed most of the comments and pushed a new commit.

[dhermes]

@nathanielmanistaatgoogle PTAL

[dhermes]

This LGTM but would like to give @nathanielmanistaatgoogle a chance to make a pass.

[nathanielmanistaatgoogle]

"Resovles" in your commit message is a typo and should be "Resolves", right?

[dhermes]

Yeah resolves is in https://help.github.com/articles/closing-issues-via-commit-messages/ but Resovles is not 😀

[dlorenc]

Thanks for catching the typo. Fixed.

[nathanielmanistaatgoogle]

(Will have other comments later; still working through the content of this change.)

[dlorenc]

Addressed feedback. PTAL @nathanielmanistaatgoogle

[nathanielmanistaatgoogle]

Aside from what we're discussing about the TODO comment I'm happy with the content of this change. Please squash commits - a code review is just a code review; there's no need for this change to be more than one piece of project history (two after GitHub does its thing).

[dlorenc]

Added the TODO and squashed. Thanks for the review!

[nathanielmanistaatgoogle]

Looks good; only waiting for tests to repass.