Allow to use an alternative token request host

Hi !

Here is a simple patch to allow the use of an alternative host if the API and the Ember app are on two different hosts, for example 'https://api.myapp.com/token'.

The idea is just to setup `serverHost` with 'https://api.myapp.com'.

The request is sent to `serverHost` + `serverTokenEndpoint`.

If `serverHost` is not set, it's an empty string by default and the request keep being relative to the current host.