Fixed token endpoint property

The specification leaves out details about how to obtain the path to the token-endpoint but suggests that it
> is typically provided in the service documentation

All authorization-server-implementations known to me are using a fixed location which cannot easily be changed at runtime.

I thus conclude that it is unneeded to supply this location with every call and started simplifying them. The most basic change for this is introduced here: a new `tokenEndpointPath` property which is set during client-initialization and used when creating the `NSMutableURLRequest`.