Add support for CURLOPT_USERNAME / _PASSWORD

Original patch by Wim Lewis (https://sourceforge.net/u/wiml/profile/).

Original patch description:

This is a really trivial patch to add support for the CURLOPT_USERNAME and
CURLOPT_PASSWORD options (and their _PROXY equivalents), which affect
the same parts of libcurl's state that CURLOPT_USERPWD does but which
don't require you to combine the username and password into one string first.
(Libcurl just immediately parses it apart again anyway.)

I've tested against libcurl-7.21.0, and looked through the source for
libcurl 7.20.0 and 7.19.5 to verify that it looks like it should still do
the right things there (in particular that curl_easy_setopt(..., NULL)
does the reasonable thing with these options).

https://sourceforge.net/p/pycurl/patches/10/
Merge pull request #32 from p-push/curlopt-username

Add support for CURLOPT_USERNAME / _PASSWORD