Support 'border' attribute for images; fix up width/height.

We now only emit width/height attributes if they are non-empty.

The border attribute is in addition to thumb/framed/frameless, according to
http://www.mediawiki.org/wiki/Help:Images#Format

However, none of border/framed/frameless should do anything if 'thumb' is
specified (by design).  Note that fact in renderThumb.

Change-Id: Ibf6e313e232403dc1f726d56886f4efe4c98a6f3
Merge "Support 'border' attribute for images; fix up width/height."