Rewrite credentials API to work with save on deploy.

Hi,

This is my pull request for the "save on deploy" part of #93.
The credentials are now fetched automatically when editing a node (as before) and only send when the user click the deploy button (with the flow).

I use a new property of the node : node._creds to store all the value needed. When the flow has been successfully send and save, the credentials information are removed from the node.

The Credentials are send into the node. There is no division between the flow and the credentials. The credentials are filtered before saving the flow.
The problem I got that lead to a separation of the methods was a corruption of the credentials file because of multiple async save done by the normal add command. What I could do is put the whole processing into the add as you asked and add a boolean as parameter to avoid the saving into the file.

About the promise, you are completely right. It's a "leftover" of the previous implementation where merge was getting the full array with all the credentials and was saving at the end. Anyway, the promise will be removed.