Add regression / integration tests for merges to master

[dhermes]

This is to prevent issues like #125 where unit tests give false sense of working code.

ISTM we'd want the following

• Python2 / Python3 - Mint a P12 Service Account access token and make a simple request
• Python2 / Python3 - Mint a JSON Key Service Account access token and make a simple request
• Python2 / Python3 - Refresh a user OAuth 2.0 token and make a simple request
• Python2 / Python3 - Mint a user OAuth 2.0 token (not possible on Travis / without human interaction)
• Python2 / Python3 - Mint a GAE / GCE Service Account access token (not possible on Travis)

[dhermes]

@anthmgoogle I'm happy to contribute this but would like to chat briefly. It will require putting a few Travis encrypted files in the repo. (See key.json.enc for example.)

Doing the actual encryption will require a project owner (and I am not a project owner).

[anthmgoogle]

I would expect that @nathanielmanistaatgoogle should be the primary owner of questions like this.

[nathanielmanistaatgoogle]

I think there's likely to be plenty here that I don't understand at the moment but I'm perfectly happy to chat about the changes you'd like to make.

[dhermes]

Great. I'll email you to set up a VC.