Added known issues section about using curl on OS X 10.9

As discussed per e-mail, when using the commands on stock OS X 10.9 there are issues with the SSL authentication using client certificates. The actual reason is, that Apple replaced OpenSSL with their own _Secure Transport Engine_, see http://curl.haxx.se/mail/archive-2013-10/0036.html

Added a small section to the README describing the issue and providing a fix (via Mac Ports or Homebrew)