blogpage: use span8 for posts without images

When a post does not have a featured image and neither an image in the content the bootstrapwp_autoset_featured_img() function returns nothing. At the same time however the span2 class that the image receives if there is an image, remains.

With this commit the_excerpt gets a span8 class if there is no image at all (not featured and no other). archive.php and perhaps author.php can be adjusted in the same way, as can index.php, but in the latter this function is not yet used.

PS only the 738eb79 file is the correct version