Fix #65. Update divider styles and include data-divider from #48

I updated the dividers used for the optgroups to now use the default
Bootstrap CSS class ``.divider``. I've also included the ``data-divider`` option. Set
``data-divider="true"`` to turn an ``option`` into a divider.

I simplified ``data-size="auto"`` - it now opens to be as tall as
possible, but with a 14px padding between the edge of the menu and the
edge of the window. Setting a number for ``data-size`` still results in mostly the same functionality, although options with ``data-divider="true"`` are not included in the count.

For example:

    <select class="selectpicker" data-size="4">
      <option>One</option>
      <option>Two</option>
      <option data-divider="true"></option>
      <option>Three</option>
      <option>Four</option>
    </select>

would show everything.

I also set a min-height on the options to take care of #65 as it was
much cleaner than a javascript solution.

I updated the reloadLi function so if an option set to selected is
appended to the select, the selectpicker text is updated to reflect the
new selected option.