Patch release

In response to the recent constructive criticism in a video review of the game engine, I decided to release a patch to answer some of the most obvious flaws that were highlighted. The most significant change is a re-write of the jumping and falling routines to use Newtonian physics, which even I can see has made a big improvement ;) I've also fixed another bug that has allowed me to increase the maximum frame rate and so hopefully reduce the lag in user input. Also I tweaked my Slime Pits level to hopefully make it a bit easier and fixed some glitches in the level design of The Sewer that the new physics highlighted.