Better authentication

There needs to be a better way to determine if there is a logged in user than watching `$scope.current_person`.

We should pull all of the authentication logic out of `$api` into a new service (`$auth` ?). I want to do something in a controller like:

``` javascript
$auth.promise.then(function(person) {
if (angular.isObject(person)) {
// person logged in, do stuff
} else if (person === false) {
// person logged out, do other stuff
}
});
```

Basically the same thing, just not relying on a `$watch`
##

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