Add fps setting and equalize animation speed

The speed of the snowflakes varies drastically between one machine/browser and the next, due to rendering speed capability. This code adds a frames per second config option and adjusts the speed of each snowflake to move the defined number of pixels at that framerate. (So if the speed is 1 and the fps is 30, each 30th of a second the snowflake will fall one pixel and make one full x-step in the 'wiggle' cosine function.)