signup email initial = account_verified_email

since we are going through the trouble of collecting the verified email
lets pre-populate the signup form.

Another thought would be to validate that the email provided matched
account_verified_email.  This would indicate that the user wants allauth
to disreguard the account_verified_email and just send a validation
email for the new provided address and make that the primary.
Merge pull request #303 from arctelix/penner-master

signup email initial = account_verified_email