Added :allow_blank configuration to number validation.

Blank fields pass validation in case the user didn't want to input their number.
Newly registered users are auto opt-in to mailing list.

Since most users miss the additional instructions to choose a contact preference, all new users are by default added to the mailing list.
Test: assert user email_contact true on creation
Merge pull request #69 from tmcyf/auto_optin

Auto opt-in to mailing list