Switch to Bootstrap 3

This PR replaces Bootstrap 2 with Bootstrap 3. This is just one strategy of many possible ones.

I think we'll be able to just drop this in as a replacement and have the vast majority of apps continue to work. For those apps that use features specific to Bootstrap 2, it might be possible to provide a compatibility layer or a global option.

Bootstrap 3 imposes a different structure for form elements (for example, each item in a checkbox group now should be wrapped in its own div). Some care will be needed to make sure that the JS code for setting values will work with BS 2 and 3, because this code sometimes searches for form elements within a very specific DOM structure.

To do:
* [ ] In `generateOptions`, make `inline` option work again.
* [ ] Fix appearance of date and date range.
* [ ] Make sure it's still possible to set input values.