A few changes which might work well for others

Hello,
  I added a few things in the process of integrating your activation plugin for a site I work on and I thought they might be useful for others so I forked your repo and made the changes there.

The changes:
* Altered activation process to handle large numbers of users (250k+) on slow hardware.
* Added a series of actions and filters to wp_new_user_notification to allow for greater control over the email sent to the user

Thanks for a handy plugin, I hope my changes will be useful for others.

--Brent