Expose entity type attributes in getters

[brettp]

I understand we don't load the attributes until the object is instantiated, but this makes doing simple things like getting a list of admin users overly complicated:

$db_prefix = elgg_get_config('dbprefix');
$admins = elgg_get_entities(array(
	'type' => 'user',
	'wheres' => "{$db_prefix}users_entity.admin = 'yes'",
	'joins' => "JOIN {$db_prefix}users_entity ON {$db_prefix}users_entity.guid = e.guid"
));

[ewinslow]

This is a duplicate but I can't find the other ticket.