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.
Merge pull request #222 from Belphemur/node-credentials

Node credentials