making radicale support authenticated user from apache

I did a small dev so Radicale can trust a user authentified by apache.

By specifying in the ```[auth]``` section ```type = apache```, Radicale will trust the authentication as passed by apache in the request (see the ```REMOTE_USER``` key).

Perks :
 * Leave the auth part to apache.
 * Apache won't execute any python for a non authentified user.
 * We'll have the authentified user name in the apache logs.