Fixed a bug with upper/lower case mismatch in data- attribute lookup.

Exhibit 2 attributes used camelcase.  new exhibit code uses as data-ex
prefix and looks up attributes using .data() .  But the spec for data-
attributes says all must be lower case, and they are being forced to
that, so the data- attribute lookup wasn't matching.  Correcting by
forcing attribute name to lowercase before lookup.

Probably the better solution long term is to convert exhibit
attributes names from camelcase to hyphenation; e.g. ex:viewClass
should become data-ex-view-class not data-ex-viewclass .  But that
involves a decision to change exhibit syntax and also requires more
substantial code changes, so I defer it.