Update user.js (added oath strategy check for email existance validation)

Don't validate if the email exists for oauth strategies.
If a user tries to connect via Twitter, the asynchronous validator for checking if a email already exists, fails. (ValidationError: Validator "Email already exists" failed for path email with value ``) -> because obviously a user who connected via oauth strategy has already an e mail with an empty string.
Merge pull request #69 from birsi/master

Update user.js (added oath strategy check for email existance validation...