Added send_email method to PasswordResetForm

Similar to Django 1.7, PasswordResetForm now has a send_mail method. This is very useful, as this method can then be overriden.

In many applications, we want to reuse password resetting functionality of Django but there is a different mechanism for sending emails (let it be a call to RESTful API, a Celery task, mailx etc).

Please let me know what do you think about this proposed change.