Modified Exhibit.getAttribute to use .attr("data-...") instead of .data()

jquery's .data() method polls data-* attributes from the tag only
once, then caches them.  So changes to the attributes (for example,
while editing the exhibit) will not be reflected in later calls to
.data().  Also, the .data() method attempts to parse the value of the
attribute and return an appropriate number, object, or array while
Exhibit.getAttribute is used in code that expects to get strings and
do its own conversion.