Make movement based on deltatime

Instead of moving '5' pixels of velocity every frame, you should multiply that by how long it takes to render one frame (delta time) and move players 5*deltatime. This should fix jumpiness between frames, and makes the player move the same speed on every computer/device.

Don't merge this yet, (this page will automatically update as I push things to my master) I'll make a comment when it's good to go :+1: