Pixel image in missed yesterday email to mark activity as read

Squashed commit of the following:

commit bd4f9cdf8c2cecc22f68d268e02a9572d1a1b538
Author: Fernando Garcia Samblas <fernando.garcia@the-cocktail.com>
Date:   Sat Jul 26 21:53:10 2014 +0200

    Reading summary email images cucumber scenario

commit 59f97ce780983ba89b9f8c17e68b547d569b475c
Author: Fernando Garcia Samblas <fernando.garcia@the-cocktail.com>
Date:   Sat Jul 26 14:12:46 2014 +0200

    Remove commented lines (ups!)

commit 05b94a3dc6e1d185ffeea6feca3a8d0b92fb3c35
Author: Fernando Garcia Samblas <fernando.garcia@the-cocktail.com>
Date:   Sat Jul 26 13:48:35 2014 +0200

    Pixel image in missed yesterday email to mark activity as read

    Send invisible single pixel image in "missed yesterday" email to mark as read automatically for people who have images enabled.
Merge branch 'mark_as_read_with_email_image' of https://github.com/nando/loomio into nando-mark_as_read_with_email_image

Conflicts:
	app/views/user_mailer/missed_yesterday.html.haml
Merge branch 'nando-mark_as_read_with_email_image'