Use SecureRandom.uuid in Component#new_uid

It may be controversial but I would prefer to not expose the hostname and to use the standard Ruby way of generating UUID. Others iCalendar libraries don't use the proposed RFC way (see [http://tools.ietf.org/html/rfc5545#section-3.8.4.7](http://tools.ietf.org/html/rfc5545#section-3.8.4.7)), so I don't think this is a big deal.