Add capability to create ServiceAccountCredentials from dict or JSON string

[nerixim]

Hi, thanks for your great job on IAP validation in Python.

It would be even better if you could get credentials without reading from a file as they have from_json and from_json_keyfile_dict class methods there.
https://oauth2client.readthedocs.io/en/latest/source/oauth2client.service_account.html#oauth2client.service_account.ServiceAccountCredentials.from_json

I could work on a PR if you're not against the idea.