Require Twisted >= 8.2.

This is already required since [a1e1382d]
[2013-09-13T22:45:08Z!h.goebel@crazy-compilers.com], but I missed updating
the requirement then. According to
http://twistedmatrix.com/trac/browser/tags/releases/twisted-11.0.0/NEWS
single byte range is supported since Twisted 8.2.0. And that version of
Coherence only supported a single byte range.
Fix and cleanup utils.downloadPage().

Since HeaderAwareHTTPDownloader == client.HTTPDownloader, there is no
need for using the first instead of the second. So this function can
be reduced to set the user agent and simply call the later one.

This also fixes a NameError: global name 'factory' is not defined.
Add comment: utils.HeaderAwareHTTPDownloader is depricated.

It was only used by utils.downloadPage, so there is no need for it
anyway.