Modified exhibit parser to ignore the (unprefixed) "role" attribute
because role has been defined as an official attribute in html5 (from
ARIA) and is appearing in many places where it will confuse exhibit.
Merge pull request #117 from karger/2014-12-26_role_attribute

Modified exhibit parser to ignore the (unprefixed) "role" attribute
modified list/cloud facets to inherit by invoking superclass
constructor instead of explicitly invoking .extend
refactored list/cloud/enumerated facets to improve code reuse
made facet._configure an instance method for enumerated facets
continued refactoring enumerated/list/cloud facet
finished refactoring enumerated/list/cloud facet