handle the 'image' attribute 'align'

Currently the 'align' attribute for images is ignored.
See http://docutils.sourceforge.net/docs/ref/rst/directives.html#image
for usage details.
The following css definitions are recommended to handle the introduced
'image-align-(left|center|right|top|middle|bottom)' classes:

  .image-align-left { display: block; margin-right: auto; }
  .image-align-center { display: block; margin-left: auto; margin-right: auto; }
  .image-align-right { display: block; margin-left: auto; }
  .image-align-top { vertical-align: top; }
  .image-align-middle { vertical-align: middle; }
  .image-align-bottom { vertical-align: bottom; }