Integrate own `localStorage` instead of relying only on Google's `sessionStorage`.

[asyarb]

Right now, this library relies on the values that Google automatically stores in sessionStorage to persist the user on page refresh, but this comes with some undesirable limitations:

• The active logged in user isn't persisted across tabs
• Closing the tab logs you out
• Refreshed accessTokens are not stored in sessionStorage and gapi provides no way to update them. A page refresh effectively wipes out any refreshed tokens.

There is a way to persist the user yourself in localStorage in userland, but it would be ideal if we could automate this behind the same persist flag.