rst: use align-attribute for images

At the moment it is impossible to change the image alignment when using restructured text on github (see #163).

The docutils ``HTMLTranslator`` is using css-classes for alignment, which are stripped by github later.
Using the HTML align attribute would be more appropriate, I think.

This pull-request adds the alignment value from restructured text in `figure` and `image` statements as `align` value to the HTML `img` tag.

I'm not sure whether this is the way it should be for github, but I think it should work well.