Fix #65. Update dividers

I updated the dividers used for the optgroups to now use the default
Bootstrap CSS class. 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
similar functionality.

I also set a min-height on the options to take care of #65 as it was
much cleaner than a javascript solution.

I also 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.
Merge pull request #67 from caseyjhol/master

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