Adding get_for_service_account_json function in credentials.

[dhermes]

Fixes #638.

@jgeewax PTAL

I thought about wrapping JSON key and PKCS12 key into one method but the JSON key is actual a JSON payload with everything needed, e.g.

{
  "private_key_id": "0996.....",
  "private_key": "-----BEGIN PRIVATE KEY-----\nMIICdgIBADA.....\n-----END PRIVATE KEY-----\n",
  "client_email": "32578400.......@developer.gserviceaccount.com",
  "client_id": "32578400.........apps.googleusercontent.com",
  "type": "service_account"
}

whereas a P12 key is just the bytes in private_key (and created for PKCS12 rather than PEM).

I don't think it's worth trying to smash together the two API surfaces:

get_for_service_account_json(private_key_path, scope=None)
get_for_service_account_p12(client_email, private_key_path, scope=None)

[coveralls]

Coverage remained the same at 100.0% when pulling 66b47b5 on dhermes:fix-638 into c6718f3 on GoogleCloudPlatform:master.

[jgeewax]

This LGTM.

That said - I think it'd be useful to add an example which would ideally go in a separate "auth" section in the docs. Do we have an open issue for that or should I create one?

[dhermes]

#633 is the issue, I actually have a branch I've been working on for awhile, I just keep getting sidetracked on it.

[dhermes]

Renamed to json_credentials_path. @tseaver can you give a quick look before I merge?

[tseaver]

LGTM after pep8 passes.

[coveralls]

Coverage remained the same at 100.0% when pulling a80f1dd on dhermes:fix-638 into f64fc55 on GoogleCloudPlatform:master.