REST API for Credentials

Adding RED.nodes.registerCredentials to register the credentials definition server sided.
Adding the property credentials in the template definition client-side.

Connecting the editor to the credential API.
I added a TODO for the validation of Credentials field.
As the other field, the developer should be able to set the credentials as required and also give a validation function.

red/nodes/index.js

@@ -36,6 +36,7 @@ module.exports = {
     addCredentials: credentials.add,
     getCredentials: credentials.get,
     deleteCredentials: credentials.delete,
+    registerCredentials: credentials.register,
     createNode: createNode,
     registerType: registry.registerType,
     getType: registry.get,