Add ability to configure a Plex Auth Token and use it for requests

For external requests to a Plex Media Server an auth token is required. This can now be configured with the following code:

```ruby
Plex.configure do |config|
  config.auth_token = "ABCDEFGH"
end
```

A custom `open` function has been implemented, which adds the Token as a HTTP Header it it has been configured. This should be a more general implementation as #11, as it prepares the gem for other settings.

Thanks