Enable cookies when fetching feeds with curl

If a feed requires cookies, curl needs to have them active in order to
access it. This is done by setting the COOKIE_JAR option.
Since we do not need the actual cookie jar, send it to /dev/null, curl
will still have a valid in-memory cookie jar.
Merge pull request #311 from fsateler/curl-cookie-jar

Enable cookies when fetching feeds with curl