Fix to use jQuery's prop rather than attr method

Needed to remain compatible with versions of jQuery from 1.6 onwards.
Fix for jQuery 1.5.2 as click() now triggers change()

Prior to v1.5.2 of jQuery, the change() event of $checkboxes did not get
called after
a click on a checkbox. As of v1.5.2, this behaviour changed, and the
click did cause
the change() event to fire, which was undesired.
Fix to work with any selector (was hardcoded to ul.tree)
Fix so that grandparents and higher refresh status

Previously, the 'refresh' event of a parent was only being called one
level up the hierarchy, meaning that checkboxes at least two levels up
the tree were not having their checked / unchecked / halfchecked status
updated on a change.
Update author info and version to 0.4