Allow choosing authentication method

Merged username-removed-400232 requested to merge feat/www-auth-from-config into master

Allow client to choose authentication mechanism based on which mechanisms are set as allowed in config. The list of allowed mechanisms is returned to the client when an illegal method or no method at all is used, through the WWW-Authenticate header.

Hawk is the only mechanism implemented at the moment, but it should now be very simple to add more.