fix iframe setup

[the ready event does not work consistently on iframes](http://stackoverflow.com/questions/205087/jquery-ready-in-a-dynamically-inserted-iframe). So any iframe-related setup code must go into the load event handler.

The problem exists e.g. in firefox but not in chrome.

This fixes only the rewriting of link targets. Other features might also be affected (e.g. relative time). The load event is only triggered after everything (including images, ...) has loaded which might be too late for presentation-centric features like relative time.