Switched to curl-multi interface for cancelling requests

Closes issue #21.

Squashed commit of the following:

commit e5f2777baec39b03e9937a5e0f1dd9deb754696f
Author: Thomas Mayer <thomas@residuum.org>
Date:   Sat Sep 21 21:37:09 2013 +0200

    Refactoring curl threading.

commit 8c7a59a75f50e892d61077bfe305289a6e1b4bd7
Author: Thomas Mayer <thomas@residuum.org>
Date:   Sat Sep 21 18:55:51 2013 +0200

    Working implementation of using multi handle.