A few changes to better work for our needs

* 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
Merge branch '1-3-1-changes' of https://github.com/bsr/User-Activation-Email into bsr-1-3-1-changes

Conflicts:
	readme.md
	readme.txt
	user-activation-email.php
Merge branch 'bsr-1-3-1-changes'

Accept pull request that adds filters in the message notification and better support for large user base.