Basic http preauthentication

Provides an optional basic http authentication.
It can be used to host private packagist service behind any kind of http authentication (htpasswd, apache with mod_ldap, etc).
This modification solves #265 without having bigger modifications on the current running instances.

The two new options are preauthenticated_provider.enabled and preauthenticated_provider.default_email_domain.
With the enabled flag you can enable http authentication listener and user provider. If you leave it false, no further modifications required.
If enabled, the userprovider is being put behind a userproviderproxy object, which proxies all the method calls to the userprovider instance, but when no user is found, it creates one.
The default_email_domain used to create an email address for the new user as email is not nullable.