Add option to persist session to localStorage

Usage:

```javascript
var user = {username: 'bob', password: 'secret'};
tidepool.login(user, {remember: true}, cb);

// This still works
tidepool.login(user, cb);

// This destroys token from localStorage
tidepool.logout(cb);
```

Token in localStorage is refreshed just like the token in memory.

Tested in Blip.

@jh-bate You might be interested in this for Clamshell :)

@cheddar Since you initially moved logged-in state to platform-client, maybe you want to have a look?