Add functions to caluculate transformation of the slides.

Add properties for transformation.

* trans-x, trans-y, trans-z
  Translate slide to each axis.

* trans-x, trans-y, trans-z
  Rotation slide to each axis.

Add customizable variables.

* org-impress-js-default-slide-class
  Default of the class attribute for the slides.

* org-impress-js-default-translation
  Default translation vector for the slides. List are corresponding to X, Y
  and Z axis.

* org-impress-js-default-rotation
  Default rotational vector for the slides. List are angles X, Y and Z axis
  in degrees.

Other changes.

* When data-rotate-z and data-rotate are given at a time, data-rotate-z will be
  prioritized.

* If any parameters of data-[xyz], data-rotate and data-rotate-[xyz] are given,
  they overwrite current coordinates or angles of the slide.
Fix an error. Symbol's value as variable is void: x
Update README.org.