Changed "page" to "slide"

This is a big one.

* Changed the word "page" to the word "slide" in the code, readme, and tests.
* Changed `slider.page()` to `slider.currentSlide()`.
* Changed the "slide" event to "move".
* Added a test for `slidesPerPage`.
* Updated tests for `on` and `off`.
* Changed the example in the readme from `.subscribe` to `.on`
* Turn tossing on when more than one slide fits in the frame.