Adds Google OAuth2 "For Installed Apps" authentication.

Hi Anton,

I have got a lot of good use out of your code, so I thought I should feed back a contribution.

I have only about six months of Python experience, so I'll be very pleased if you spot improvements. For example, I wanted to put the test stuff in your test directory, but couldn't see how to set up the imports correctly.  How do you do it?

As you will see, I have tried very hard to ensure that you have an absolute minimum of changes to review; it's almost 100% optional additions.

Sincere regards,
MHB