Adds Digest Authentication

Adds Digest Authentication to work with atleast Baikal Server(I am sure
there are more).
Defaults to basic auth. Add , :authtype => 'digest' to your
Client::new. Declaring 'basic' is also valid

Also changed when creating or finding events the time generated is now
changed to UTC and added Seconds as per the specification

Tested to work with Baikal(Create Event, Find Event, Find Events) May
still be some small bugs.