Adaptive Difficulty: Level-up / Level-down should occur based on player performance

[aarontabor]

For those experienced with the game controls, playing lower levels quickly becomes boring. To address this issue, we can intelligently increase/decrease the current level (i.e., platform speed) based on the player's in-game performance.

To accomplish this, we first divide the playing area into 3 logical regions:

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|   Danger Zone   |
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|    Neutral Zone |
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|     Happy Zone  |
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By tracking the percentage of time that a player spends in each of these regions, we can estimate their in-game performance. For example, a player spending the majority of their time in the danger zone can be assumed to be performing poorly, and should therefore be level-ed down. Similarly, a player spending the majority of their time in the Happy Zone can be considered to be performing well, and should be level-ed up.

[aarontabor]

see #60