Add ability to pass `response` to `HTTPError()`

Just a little refactoring, but it seems nicer to me to be able to pass
the response when constructing the `HTTPError` instance instead of
constructing it and then changing the member variable.
Merge branch 'httperror_init' of git://github.com/dmedvinsky/requests

Conflicts:
	AUTHORS.rst
	test_requests.py
Merge remote-tracking branch 'origin/master'