New method - checkWhether - to check specified class attribute on a node

But really, it helps us check the class attributes on tabs
Special handling for nopurge-* class attributes
[big one]: Rationalized class attributes to set on tabs

The following are the only class attributes that control whether purging, ajax
calls etc are invoked on tab-click

  1. nopurge-on-show
  2. nopurge-on-inactive
  3. nopurge-on-disabled
  4. nopurge-ever

These can be set either on
  1. ul.nav-tabs > li > a OR
  2. ul.nav-tabs itself - in which case all ul > li > a are said to have the
     same class attribute