Add optional username feature to account information pane

Basically, creating an order based on email address means the email address is now exposed for that account (on the user page). To keep the email addresses private (which is what Drupal's standard register form says it does, so this would be consistent) it would be great to have an option similar to the double email field to enable a username field that is used to generate the account instead (with validation to ensure no duplicates, etc).