Allow auth view to be served on non-default domain

As different parts of the app may be served on different subdomains - e.g. browsable views and API - if would be great if flask-jwt provided an option to specify desired domain for auth view.

Additionally:
+ Removed second layer of defaults for 'JWT_AUTH*' configuration parameters

    (as I don't see how those parameters may stay undefined by that point)
+ Removed check for 'JWT_AUTH*' defined

    (as the extension doesn't seem to be very useful without auth view)