Fix vertical alignment on boxes

Previously the box icon was a floated image, so if the box was more than
a couple lines, the alignment of the icon was ugly. Now the icon is set
to float left and be centered vertically. This fixes #24
fixed an array bug

when extracting the box atts I assumed that $atts would be an array,
which was an improper assumption to make. I also bundled in a few other
little fixes as well.
update readme and plugin version