Adds option for popovers and tooltips so you can easily add a custom class

Created by: theodorton

Examples:

$(selector).popover({
  'class': 'my-custom-class'
});

$(selector).popover
  class: 'my-custom-class'

Should result in:

<div class="popover my-custom-class">...</div>

May not work if you're also setting the (undocumented) template-option, but I'd say it's redundant to assign both.

Tested and documented.