Add destroy method and improve snowflake calculations. Add demo page.

• Add a method to destroy the snowfall and remove the canvas element;
• Change some functions to arrow functions for consistency;
• Simplify some conditions for checking the visibility and position of the snowflakes;
• Change some calculations for the speed and size of the snowflakes;
• Fix some minor issues with the canvas resizing and appending;
• Improve the cross-browser compatibility and performance of the script;
• Add a demo page for demonstrating the capabilities of the script.

First release of Snowfall.js

JavaScript library for creating and animating snowflakes on a web page.