Use live() instead of on() to restore jQuery < 1.7 compatibility.

The bugfix introduced in 4cd7e08 uses `$().on()`, which only exists in jQuery >= 1.7.

Since Plone 4.2 only ships with jQuery 1.4.4, this effectively broke Plone 4.2 compatibility. In this PR I replace the use of `on()` with `live()` in order to restore compatibility with jQuery 1.7.