Make lxml clean tree available for user modifications

[hush-hush]

The cleaned lxml object wasn't available outside readability.

It very useful to ask readability to extract and clean an article and then modify the lxml tree before generating a html text stream.

An example to update image link:

d = readability.Document(html)
d.summary(True)
for i in d.html.findall(".//img"):
        # ... some modification ...
summary = d.get_clean_html()

[buriy]

Applied.