[wip] Refactoring credentials into foursquare-credentials

Refactoring the credentials side of the swarm node out into its own node (https://github.com/node-red/node-red-web-nodes/issues/41).

@knolleary - although I've marked this pull request as [wip] it's all ready to go once pull request https://github.com/node-red/node-red/pull/449 has been merged. The failing tests which appear against this refactoring pull request will be fixed when the changes are made to the main node-red test helper.