Added events to affix to resolve issue #6421

Added 4 events, pin, pinned, unpin, and unpinned to resolve [#6421](https://github.com/twitter/bootstrap/issues/6421). Adding events to affix required extracting "affix" and "unaffix" into their own methods pin and unpin.

Should something like this be broken into multiple commits to make it easier to see changes? Sorry, still new to contributing with github.