Renamed @pane an @slider to @yPane and @ySlider

For allow future compatibility with horizontal slider
Replaced paneClass to paneClassY to allow horizontal pane
Fixed pane style name
Added static vars for horizontal pane
Added BROWSER_SCROLLBAR_HEIGHT static var
Renamed preventScrolling to preventVerticalScrolling
Renamed some methods and vars
Complete initial support for horizontal scrollbar.
Fixed typo
Added dynamic width for scrolling panes

Depending if is showing both or not.
Adapted tests to new pane class names.
Added methods for maintain compatibility with previous version
Updated API for compatibility with previous version.
Updated README with the horizontal scrolling features.