Fixed #245 -- The first ``AccordionGroup`` can now be set ``active=False...

- Changed it so that if the user initializes the first ``AccordionGroup`` with the ``active=False`` kwarg, that first group will not be initially visible at page load.
- Kept the original default behavior, which renders the first group when the ``active`` kwarg is not set at all. Therefore, the change should be fully backwards compatible.
- Unit test written for new functionality.
- All previous unit tests pass.