Improved comments and organization, optimized functions

- The code for each player is now separated more clearly. The only interdependency between them now should be in the trigger condition for addGarbage() calls.
- The game loop in tetris() has been significantly optimized such that the code does almost nothing during any cycle for which no input is entered. This may cause noticeable inconsistency in cycle timing when run on the Raspberry Pi, but since it is relatively predictable it may be possible to resolve such an issue with a simple, short usleep call.
- Several other functions were slightly optimized, particularly importPiece().