This release turns scale option support back, and sets default scale to 1

Thanks to #52 by zero41120 there is no need to adapt fabric.js for using in this plugin anymore

upgraded fabricjs to 4.0.0-rc.1 by @tdiprima

This release uses OpenSeadragon viewport coordinates and is suitable for multiple images. There is new parameter in creating fabricjsOverlay - options, options.scale is used to define fabric area, so objects can be created accordingly. Also this version includes modified by @Faibk fabric library that accepts pointer events, so it should work in Chrome 55+

Updated positioning code. Now it works in fullscreen, and with offset OSD