Fixes #21

Previously, --no-gateway wasn't being honored correctly. Instead of not
creating a route as intended, the correct representation of a gateway
for a subnet, we were creating a default route with CIDR 0.0.0.0/0 and
NULL gateway. This patch allows gateway_ip == NULL to be passed and
handled appropriately.
Merge pull request #22 from Cerberus98/issue21

Fixes #21