src/Tweeper.php: add a retry mechanism for cURL sessions

Sometimes the connection to a remote host may stall and a resource
cannot be retrieved. This makes Tweeper hang for a very long time which
can be annoying for users.

Setting a shorter timeout and a retry mechanism usually works around the
problem allowing the resource to be retrieved eventually.

Implement such a mechanism by adding curlExec() method and while at it
move non-curl related messages outside of getUrlContents() and
getUrlInfo() to give the user a better understanding of what actually
failed when even the retry mechanism was not able to retrieve the
resource.