Add modal service that uses bootstrap js for show/hide.

* Add 'bootstrapModal' module that provides a service to generate modals that
  use bootstrap's js for show/hide.  Inspiration for the logic was provided by
  angular-ui's boostrap modal implementation.

* Remove outer two container divs in task_modal.html.  These are now provided by
  the modal service.

* Remove taskModal directive.

* Add taskModalCtrl controller for task create/edit modal.