* New SessionState to consolidate email, access token and refresh token
* split ServeHttp into individual methods
* log on session renewal
* log on access token refresh
* refactor cookie encription/decription and session state serialization
This is the first step towards genericizing the google_auth_proxy to support
OAuth2 providers other than Google as discussed in #65. The `api` package will
enable multiple providers to use the same `api.Request()` implementation.