switch to 'valid_email' gem for email validation

the current regexp-based validation still lets through duff emails
of the form 'abc @d.com'. The valid_email gem (https://github.com/hallelujah/valid_email)
uses the 'mail' gem under the hood instead of regexps.
better email validation message
Merge remote-tracking branch 'origin/remove-regexp-based-email-validation'