Re-rewrites ass_send_multipart_email().  Fixes #17.

Work was done in commit 972ce9f to refactor the digest email function
to use wp_mail().

However, wp_mail() has a bug:
http://core.trac.wordpress.org/ticket/18493#comment:13

It does not clear its $phpmailer global properties, which can lead to
users receiving the wrong email content.  This issue has been reported
by multiple GES users:
https://wordpress.org/support/topic/daily-digest-email-missent

This commit clears the email content properties in the $phpmailer
global object, so we can ensure that no cached values from previous
digest email sendouts are used.

This commit also removes our custom email 'From' header hooks after
the email is sent.

Thanks to defunctlife for testing this code on a production site.