Fix the function that joins base urls and relative / absolute URLs.

The Python standard version of this function is more capable than the previous
version. The previous version could not join URLs of the form
//www.mydomain.com/foo with the base URL. See a live example of this problem
in the feed http://www.infojobs.net/trabajos.rss/kw_candidate/
Merge pull request #6 from e98cuenc/fixurl

Fix the function that joins base urls and relative / absolute URLs.