Adds 'GooglePlus::AccessToken', to exchange refresh_tokens.

Hi,

I added an AccessToken class, which handles the exchange of refresh tokens for access_tokens.

I implemented a client that needs offline access to user resources. This can only be achieved through refresh token exchanges, because the provided access_tokens are short lived tokens (1 hour).
I thought it would be nice if the gem provided a way to handle this.

I also added fixture data for testing. I don't think the tests should be actually hitting the api.
By their structure, the tests look like unit tests. As unit tests, they should trust RestClient and Google's api, and only be concern about the logic inside the methods being tested.
I'm willing to do the work of changing the tests to use fixture data, but I wanted to hear your opinion first.

Fito.