Support for Digest Authentication

Added support for Digest Authentication (username / password). The user can now choose between a Token Authentication (default) and a Digest Authentication, when logging in.

This was necessary for integrating the SKOSjs app with a Virtuoso instance. If SKOSjs is to be used against a Virtuoso instance, you need to use Digest Authentication, against a /sparql-auth endpoint.

A running instance can be tested here: http://fusepool.openlinksw.com/DAV/SKOSjs/index.html. Ping me for the password.