bugfix: should check the real activeElement in framed website

when a web page has frames, document.activeElement may return FRAME instead of
a lower level element. It should walk down the path when activeElement is a
FRAME.
update doc
update changelog
fix bug 293: should match img.src using img.getAttribute('src')
fix bug 299 not working on http://bbs.xineurope.com/thread-785643-4-1.html

for non discuz forum, do not produce a js error.
for some discuz forum that does not have a generator, also check
content.discuzVersion
v1.3.7