renamed hostname to host in the routing system (closes #6775)

As explained in #6775, this has been done for the following reasons:

1. It's also Request::getHost()
2. The term hostname has been obsoleted in
http://tools.ietf.org/html/rfc3986#appendix-D.2 and uses the host only
3. hostname in the RFC was defined as the registered domain name, but we
probably also want to match IP-Adresses with the pattern which is the
host = IP-literal / IPv4address / reg-name for.