Add -Dhttp support.

* bzrlib/transport/http/_urllib2_wrappers.py:
(HTTPConnection.__init__): Report the host we are about to connect
to if -Dhttp is used.
(AbstractHTTPHandler.do_open): Report requests and
responses (including headers) if -Dhttp is used.

* bzrlib/transport/http/_urllib.py: Fix some imports.
(HttpTransport_urllib._perform): Delete one mutter call since
-Dhttp provides better information.

* bzrlib/transport/http/_pycurl.py:
Fix some imports.
(PyCurlTransport._set_curl_options): Activate verbose output if
-Dhttp is used. Unfortunately this goes straight to stderr instead
of .bzr.log (libcurl provides an option but pycurl does not
implement it), but since we are debugging, I think it's
acceptable.

* bzrlib/transport/http/__init__.py:
(HttpTransportBase._coalesce_readv): Add a comment about the
servers that return the whole file ignoring the Ranges header.

* bzrlib/help_topics.py:
(_global_options): Add http.

* bzrlib/debug.py: 
Add 'http'.