Add support for x-auth-token

X-Auth-Token implementation to enable stateless configuration of jhipster.

Switching between the generator options is done explicitely (as shown below) for better code understanding (cookie/token/xauth).
> if (this.authenticationType == 'cookie' || this.authenticationType == 'token')
is more explicit than
> if (this.authenticationType != 'xauth')
For more clarity, 'token' could also be renamed to 'oauth2'

Implement #892