Operations to read attribute group information

See "Part III Other possible things to do" in http://lists.jboss.org/pipermail/wildfly-dev/2014-December/003414.html, and Heiko Braun's response thereto.

1) Add global operation "read-attribute-group(name=<name_of_group>) which functions similarly to read-resource, but only includes in the response those attributes defined as belonging to attribute group "name_of_group".

Besides, the "name" parameter above, parameters and effect thereof should be consistent with "read-resource", except that any parameters dealing with inclusion of child resources should be excluded.

2) Add global operation "read-attribute-group-names()", which will return a ModelType.LIST whose elements are ModelType.STRING, each element being the name of an attribute-group with which an attribute is associated. Any name only appears once, no entry for the null group to which most attributes belong, response is an empty list if no attribute groups are defined.