WIP: slidebox

Test is in test/html/slidebox.html

Benefits of refactor:

- Only current, previous, and next element are ever in the DOM
- Only the scope of the current element is ever connected to $watches, events, etc
- Abstraction of swiping between elements is open for tabs with almost no work, and also sidemenu and views with a bit of work.

TODO

- [x] Lower the transitionDuration depending on user's drag velocity.
- [x] Finish documentation
- [x] Fix CSS height problem
- [x] Unit test utils/list
- [x] Cover case with only two slides and previous