Testing for GridModel
Added tests for GridModel initialization, rotating, querying for start and goal, connection finding. Added methods to GridGenerator and GridModel to ease testing by initializing from a string and grid, respectively. Tested interface for querying CellModel for openings directly and indirectly. Changed ViewController initialization of level to query for level 0 to prevent crash while only level 0 is defined.
Tested: iOS Simulator.
Merge remote-tracking branch 'origin/gridModel' into gridModel