Add ability to use alternative icons

Add the ability to use alternative icons like FontAwesome. For example:

    $('#demo-builder').queryBuilder({
      icons : {
        add_group: 'fa fa-plus-circle',
        add_rule: 'fa fa-plus',
        remove: 'fa fa-times',
        sort: 'fa fa-sort'
      },
      ...
    });
Separate remove icons into remove_rule and remove_group
Merge pull request #32 from jasonhancock/icons

Add ability to use alternative icons