[node] Email validation flow

[bruderstein]

In the user signup (auth.js, /api/auth/signup), we need to send an email with a random validation link (could be /validate?userid=23423&validity=.....). This link should probably contain some random identifier for the user (can add a guid to the user, for example), and then add a validation code. The validation code that is stored should be hashed (with something like SHA256), and the original code sent in the email.

The /validate route then needs to set the user to validated (set a value in the email doc), if hashing the validation code sent matches what is stored in CouchDB.

The email system should be added first (issue #7 )