Add destroy method for Flexslider

By popular demand.

- Destroys FS generated markup and events.
- Removes FS generated styles (although be warned it _could_ clobber a user's inline styles if they exist).
- Supports multiple instances of Flexslider.
- Includes demo.
- Was originally authored for the master branch and I did my best to work off of the newer development code. You may want to keep a close eye on that.