[RFC] Allow usage of google api client 2.0

[cedricziel]

It would be great if the consumer of the package could decide, which version of the Google API Client libraries he/she/it wants to use. The signatures of the relevant classes should be stable and didn't diverge.

We in particular are excited for new authentication methods for our API Client integration https://github.com/websightgmbh/l5-google-client

To allow graceful transition, I propose a new major version of your package (which would block composer from updating the api-client libraries from 1.x in older / existing projects). This new version could as well get the change from #4 , which may interfere with you current production environment.

I will pull-request the change to your package but leave the ticket here for discussion.

Thank you in advance!