Better IP Masq support and host-gw backend added

1. --ip-masq now works for all backends. Previously it was only
   working properly for UDP backend.
2. Addition of new "host-gw" backend -- thank you, Angus Lees. This
   new backend avoids encapsulation and uses the host's interface as a
   gateway. As a subnet is added or removed, each flannel daemon will
   manipulate the host's routing table to reflect the change. As an
   example, if a host that has an IP 192.168.1.5 adds a flannel subnet
   10.1.8.0/24, the following route would be added to each host:
   10.1.8.0/24 via 192.168.1.5 dev eth0