E-mail input validation

[paravoid]

When an e-mail address is submitted (either during the registration or from the profile menu), it should be checked for validity, i.e. \w+@\w+\.\w+ This will avoid common typo mistakes (missing a dot, a @ or the inclusion of whitespace). It will also correct the current behavior in which a user can have multiple email addresses if he separates them with a comma.