option to use google cred as env variable

[julbrs]

Hello

I am using a hubot chatbot on a Google Hangouts Chat system and we are using it on a Heroku-like system, with no way to declare a GOOGLE_APPLICATION_CREDENTIALS=cred.json.

So I propose the following to allow using Heroku/Dokku/nameit, using the environment variable GOOGLE_APPLICATION_CREDENTIALS_DATA that contains the content of the json file. See the README file for more info.

Let me know if you need additional info !

Thanks for the adapter !