1.2.1.0 Balancing tweaks

After playing the game with a few other plays I've made a couple of small tweaks to hopefully balance the gameplay a little better.

Changes include:

• Slowing down the sheep once they're in the pen area to discourage players from quickly sweeping out their rival sheep.
• Give the power ups different probabilities for appearing. Speed up is most common followed by freeze. Add sheep is least likely to appear.