AccessToken additions

- It now serializable, as it the main credential data to call graph, it should be storable.

I have a problem with `isValid()`, I think sometimes you want check validity only with expires, without made an addition Graph API call to `/debug_token`.
I propose to have two access token classes:
- a `SimpleAccessToken` class like the current one, without full validity checks
- an `AccessToken` class than extends above one, and perform the graph call in constructor, and have accessors to all new useful data like user ID, scope, ... that are deeply associated to the access token.

About naming, I'm not sure, it can be `AccessToken` for the simple one and `FullAccessToken` for the second one.

@gfosco, @SammyK some thoughts ?