Added module Elm_slider.
Added elm_slider_add.
Added format functions in Elm_slider.
Added elm_slider_horizontal_set/get.
Added elm_slider_min_max_set/get.
Added elm_slider_inverted_set/get.
Added events in Elm_slider.
Added example slider_example.ml.
Fixed unit/indicator format for sliders/progressbars.
Updated README.