Initial version of the script for asynchronous ad-fetching from OpenX

Initial version of the script for asynchronous ad-fetching from OpenX

openx.js replaces the OpenX-script spcjs.php. It is modified in such a
way, that it can be called at the bottom of the HTML-page, just before the
closing </body>-tag, so that the ads are loaded, when the rendering of the
page is already completed and, thus, do not block the rendering of the
page.

Implementation-details:
 * openx.js searches via jQery for DOM-elements, that are marked with the
   CSS-class "oa". It fetches banners for all found elements (which banners
   are to fetch is encoded in a second CSS-class).
 * openx.js takes seperate steps to
   1. fetch the banners,
   2. render them in the page and
   3. show them.
   This is necessary, because most of the second-party ad-scripts work
   with document.write(), so that the created banners first become
   accessable via the DOM, when the script returns.
 * Banners are rendert first into a div-block marked with "display:none".
   They are moved to their final destination afterwards. This is necessary,
   because "document.write()" always writes to the actual position of the
   HTML-document, that is the position, where the browser found the
   openx.js-script he is just executing. Hence, all ad-scripts that call
   "document.write()" itself, would render their banner at the bottom of
   the page, albeit the script itself was appended to the right DOM
   -element. The Banners are renderd in a hidden div-block, because it has
   shown, that some ad-scripts produce errors, when they are moved, so that
   the ad was visible in two places afterwards: the div-block at the end of
   the page and at the intended position.

TODO:
 * Not tested with IE yet!