Adds current user information to notifier.js

This adds the current-user part of the Airbrake notification API v2.4.

To configure, add this to your Airbrake configuration:

    Airbrake.setCurrentUser({
      id: '1',
      name: 'some one',
      email: 'user@example.org'
    });

An alternative would be to look for a `current_user` variable on the global `window` object, but this seemed more in keeping with the rest of the config implementation.