Adding Bullet Shielding

Yes, I've been thinking about that for awhile, even before [[ShieldBot]] (which ranks #3 in vote) is created. Anyway, doing bullet shielding would mask some improvements in both gun and movement, therefore I decided not to add it until the last (probably) version.

ShieldBot shares a lot of code with ScalarBot (actually all the framework stuffs are from ShieldBot, and that framework is designed for multi-mode, especially bullet shielding), which makes adding a bullet shield painless (just copy the shield module into ScalarBot, then, done).