Add warning when net.ipv4.ip_forwarding = 0

Added warnings to api.go, container.go, commands.go, and runtime.go
Also updated APIInfo to return whether IPv4Forwarding is enabled
Add Colin Rice to AUTHORS file
Fix reversed IPv4Forwarding check in api.go
Merge pull request #1395 from c00w/490-warn-ipv4forwarding-disabled

* Runtime: Add warning when net.ipv4.ip_forwarding = 0