Added open/close custom events (not callbacks), README, LICENSE and bug fixes

I hope this pull request is easier to do. I've added custom event triggers to both modal open and close from the modal_id, cleaned up quotes and spaces, fixed some bugs (multiple overlays, added preventDefaults, etc), fleshed out the README with How To and changelog. Finally, I added a LICENSE file.

Great job on the plugin, lean is the way to go!