Node credentials

As discussed in the issue #93 here is the pull request following all your recommendation. 

I just thought that instead of creating a **RED.nodes.registerCredentials**, changing the **RED.nodes.registerType** to accept a third parameter that represent the definition of the credentials.

Also, I made the object Node load automatically its credentials into its attribute **this.credentials**

I took your sample node and created a new one with the support of credentials.