feat(jqm-class): add jqm-*-class directives

Helper directives which add a class based on the current
theme & scope.

* jqm-position-class directive - adds class based on position
* jqm-active-class directive - adds class based on touch-state
* jqm-once-class directive - adds a class once after interpreting (no
 watch)
* removes jqm-theme-class directive

based on discussion in #48