Small improvements

Thanks for the great module.

I suggest to add these improvements (see the attached patch):
1. Better and more correct plain text email formatting.
2. Checkbox 'Send node to subscribers' on node edit form is checked by default for node creation only.
3. Construction correct email address when 'From name' is empty.
4. Subscription form button label for anonymous users should be 'Subscribe'.

BTW, the logic in_postsubscribe_format_html() function is not quite correct. Could you fix it? http://drupal.org/project/pathologic or similar modules can provide the code.