allow shared token authentication of TriggerRemoteBuild

the TriggerRemoteBuild checks that the ip address of the source repository matches the IP adress of the build trigger.

The architecture of GitHub.com has changed since they moved from the http://www.engineyard.com/ to rackspace. The ip address is no longer the same as the repository (in fact it changes with each commit -have seen dozens of different ip addresses)

Would be nice if we could use an optional shared token to authenticate for cases where we can't use the ip address.

It would be nice if the trigger was a plug-in that we could tailor for each repository environment.