kadenz: implemented Kadenz.prevStep method

Kadenz.prevStep() method is called to rewind slides, e.g. when arrow up
is pressed.  The behavior of kadenz.prevStep() is equal to
Kadenz.skipToPrevPage().  Therefore the slides will be skipped to
previous page to rewind slides.
kadenz.io : replace event registration as adding callback

Before the modification, the event callback method is registered as
following:
  window.onload = function(e) {};
By the above description, it is able to register only one callback
method to catch the event.  So all event registration is replaced as
adding a callback to keep availability as following:
  window.addEventListener("load", function(e){});
kadenz: specification change in keyframe

Changed specification in keyframe.  Before specification change, the
animation is specified with raw css simply but redundantly :

    <keyframe target="box4" css="width: 96px;
                                 transition-property: width, background-color;
                                 transition-duration: 1s;
                                 background-color:red" />

Now the redundant description have been improved to describe in
attributes as following:

    <keyframe target="box4" duration="1s" width="96px" background-color="red" />

The animation is based on css transition, which is unchanged, but some
description had been reduced.  First of the change is that the duration
is specified in attribute directly.  Other modification is the
transition-property is removed.  The transition-property will be set
from key of attributes on animation.

Another modification is the following.
- Neutral style of the element on page transition
- Modified tutorial to change the explanation about element animation
kadenz.pager: added new module Kadenz.Pager

Added new module Kadenz.Pager to project. Kadenz.Pager is presenter
assist tools.  When number key pressed on the presentation, the pager
will be shown.  The Kadnez.Pager is useful to search page and jump to
specified page.

The pager is at the experimental stage as only jumping with text box.
To complete the pager and to implement more module, it is needed to
review the design of core modules.
kadenz: fixed bug with hash in url

When the page is loaded with page number in hush, the page specified in
hush have became to open the page specified in hush.

TODO: Divide to call a member method of window, and core implementation
The process in window.location is implemented in Kadenz.initPages()
method now.  It is needed to divide the process around window and core
modules.
kadenz.pagenum: added new module kadenz.pagenum

The kadenz.pagenum module is page number of the slide.  The page number
will be added into element which has specified class name or default
class name ("pagenum").

The feature to folio to the page is implemented in kadenz.base and core
module, but is contained bugs.  The above codes have been reconstructed
to Kadenz.pagenum and fixed bugs.