Fixed slipping (partially), added gems, added camera movement

After 2 days of depressed mood, I fixed slipping when fallible blocks fall to slippery surfaces. The logic is still unsound and sometimes boulders behave weirdly, for example when multiple boulders fall in parallel, with unsynchronized starting frame. Original game probably had movement steps in sync.

Finally, added edible gems (or diamonds?) which is primary nutrition of our diggie-froggie.

Added camera movement, so you can walk around for more than single screen.

Things to do before the end of submissions:

• Implement death. Currently froggie is immortal and his head is boulder and gem-proof.
• Add intro screen. This is the most important part of every game.
• Add elevator on which froggie rides away after intaking required ration of gems. I'm too lazy at this point of time to implement multiple levels.